# Metformin: Unlocking the Secrets of the World's Most Prescribed Diabetes Medication and Its Potential for Treating Other Diseases Metformin has been the world's leading type 2 diabetes medication for six decades, used daily by more than 200 million patients, yet scientists are still uncovering surprising new details about exactly how it works. This review highlights major paradigm shifts in our understanding: metformin acts not just on the liver but also powerfully on the gastrointestinal tract, gut bacteria, immune cells, and even brown fat, with new molecular targets identified at the lysosome. The researchers also explore promising — and sometimes disappointing — evidence for repurposing metformin in cancer, inflammatory diseases, age-related conditions, and COVID-19, while emphasizing the importance of using clinically relevant drug concentrations in laboratory studies. # Metformin: Unlocking the Secrets of the World's Most Prescribed Diabetes Medication and Its Potential for Treating Other Diseases ## Table of Contents - Key Points - Why This Research Matters - Metformin at a Glance: What You Need to Know - How This Review Was Conducted - Where Metformin Works: A New Map of Target Organs - How Metformin Works at the Cellular Level - Metformin and Your Gut Bacteria - Metformin and the Immune System - Metformin, GDF15, and Body Weight Control - Metformin and COVID-19: What Clinical Trials Found - Repurposing Metformin: Cancer, Aging, and Inflammation - Clinical Implications: What This Means for Patients - Limitations of the Research - Recommendations for Patients - Frequently Asked Questions - Source Information ## Key Points - Metformin works in the gut, liver, brown fat, and immune cells, not only the liver. - Clinical trials showed no benefit of metformin for early COVID-19 treatment. - Metformin may cause modest weight loss via GDF15, but effects vary. - In pregnancy, long-term child growth was similar with or without metformin exposure. - New research identifies lysosomes as a target at therapeutic drug concentrations. ## Why This Research Matters Metformin (1,1-dimethylbiguanide hydrochloride) has been the most commonly used glucose-lowering medication for the past 60 years. It is currently the first-line treatment for individuals newly diagnosed with type 2 diabetes mellitus (T2DM) in most clinical guidelines worldwide. Despite its widespread use, scientists admit that the mechanisms underlying its therapeutic action are complex and still not fully understood. The drug has an impressive track record. Its long-term safety and efficacy are well documented, and it offers several important advantages: a low risk of hypoglycaemia (dangerously low blood sugar), cardiovascular benefits, mortality benefits, additive or synergistic effects when combined with other medications, low cost, and wide availability. Metformin also moderately reduces body weight gain, possibly through a hormone called growth differentiation factor 15 (GDF15). Perhaps most striking is the scale of its use: **over 200 million patients worldwide take metformin daily**, either alone or in combination with sulfonylureas or dipeptidyl peptidase 4 (DPP-4) inhibitors. The usual dose is **0.5–2.5 grams per day** (as immediate-release or extended-release tablets), which provides effective long-term blood sugar control with notable improvements in HbA1c (a measure of average blood sugar over 2–3 months). ## Metformin at a Glance: What You Need to Know Metformin belongs to a class of drugs called biguanides — a synthetic compound made of two coupled molecules of guanidine. Here are the essential pharmacological facts that shape how the drug behaves in your body: - **Absorption:** Metformin is mainly absorbed in the upper small intestine and exhibits "flip–flop" pharmacokinetics (meaning absorption is slower than elimination), resulting in only limited oral bioavailability. - **Chemical properties:** It is a basic, water-loving (hydrophilic) drug with a pKa value of 11.5, existing as an organic cation at normal body pH. - **Transport:** The drug's distribution depends on specific transporter proteins, including organic cation transporters (OCTs), plasma membrane monoamine transporter (PMAT), and multidrug and toxin extrusion (MATE) proteins. - **Half-life:** Metformin's half-life in plasma is short — **2 to 6 hours** — leading to a steady-state plasma concentration of approximately **4–15 μM (0.5–2.0 μg/ml)** in patients with T2DM. - **Tissue accumulation:** Metformin accumulates in multiple tissues, including tumours, at concentrations much higher than those in plasma. This raises important questions about which concentrations are truly relevant for laboratory studies. Using advanced imaging with 11C-labelled metformin PET–CT scans in humans, researchers found the highest drug uptake in the gastrointestinal tract, liver, and kidneys. However, current imaging resolution is not fine enough to determine exactly where within cells metformin concentrates. ## How This Review Was Conducted This is a comprehensive review article published in *Nature Reviews Endocrinology*, a leading peer-reviewed journal. The authors — Marc Foretz, Bruno Guigas, and Benoit Viollet from Université Paris Cité, the CNRS, Inserm, Institut Cochin in Paris, France, and Leiden University Medical Center in the Netherlands — analyzed and synthesized findings from numerous studies published in recent years. The review builds on a previous article the same team published in 2019 in the same journal. For this update, they focused on selected papers published since then, most of which involved preclinical (animal and cell) models, while also incorporating human clinical studies examining the gut, immune system, weight regulation, COVID-19 outcomes, and potential new uses for the drug. ## Where Metformin Works: A New Map of Target Organs For decades, the liver was considered the major organ responsible for metformin's blood sugar-lowering effects, primarily by reducing the liver's production of glucose (hepatic gluconeogenesis). However, emerging evidence tells a more complex and fascinating story. ### The Liver: An Important Player, But Not the Only One Early research highlighted the liver as the primary site of metformin action for controlling hepatic glucose production, acting through both AMP-activated protein kinase (AMPK)-dependent and AMPK-independent mechanisms. But clinical studies in people with recent-onset T2DM (duration of diabetes less than 50 months) and in non-diabetic control participants revealed something surprising: metformin was associated with an *increase* in the body's own glucose production (endogenous glucose production). This finding indicates that metformin's glucose-lowering action is **not entirely governed by reducing liver glucose production**. ### The Gastrointestinal Tract: A Hidden Powerhouse Over the past few years, the gastrointestinal tract has become a major focus of attention as an additional — or even alternative — site of metformin action. Several lines of evidence support this shift. First, metformin accumulates in the gut at concentrations **30 to 300 times greater than those in plasma**, making the gut an important metformin reservoir in both humans and animal models. Researchers have proposed a "sponge" hypothesis to explain the slow, dose-dependent absorption of the drug along the gastrointestinal tract. In humans, metformin enters intestinal cells (enterocytes) through a saturable (limited-capacity) transport process on the apical (lumen-facing) side, but its release through the basolateral (blood-facing) side is inefficient due to the absence of efflux transporters. This means the drug becomes sequestered and concentrated within the intestinal cells. Some metformin in the intestinal lumen may also pass between cells (paracellular transport) to reach the bloodstream. In rodents and minipigs — and confirmed in patients with T2DM — metformin inhibits the intestinal absorption of dietary glucose. PET–CT imaging with 18F-labelled fluorodeoxyglucose (FDG, a non-metabolizable glucose analogue) showed accumulation of this tracer in the intestinal lumen of diabetic Goto–Kakizaki rats after a single oral dose of metformin. The mechanism involves a transient decrease in the abundance of **sodium–glucose transporter 1 (SGLT1)** at the apical membrane of enterocytes in the jejunum. Remarkably, the reduction in postprandial (after-meal) glucose response from a single metformin dose was completely abolished in mice lacking SGLT1 — but not in mice lacking glucose transporter 2 (GLUT2). Additional studies found that a single dose of metformin into the jejunum of minipigs reduced intestinal glucose absorption while increasing release of **glucagon-like peptide 1 (GLP1)**, a hormone that stimulates insulin secretion. This suggests that delayed glucose absorption — and the exposure of glucose to more distal regions of the intestine — is sufficient to trigger GLP1 secretion after an oral glucose load. Human studies tell a similar story. A single administration of metformin into the proximal and distal small intestine markedly reduced the glycaemic (blood sugar) response to oral glucose in patients with T2DM, along with enhanced GLP1 secretion. Using newly developed 18F-FDG PET–MRI techniques, researchers demonstrated a **dose-dependent metformin-induced accumulation of glucose tracer** in both the intestinal wall and the luminal space of the ileum and colon of T2DM patients taking metformin. Animal experiments with 18F-FDG PET in mice fed a high-fat diet (HFD) showed that metformin given orally reduced the transport of glucose from the proximal small intestine lumen into the circulation. Interestingly, PET imaging also showed increased glucose uptake from the bloodstream into the gastrointestinal system after acute oral administration of metformin — meaning the gut acts as a "glucose sink," pulling glucose out of the blood. Here's the proposed sequence of events: metformin promotes glucose uptake into enterocytes from both the bloodstream (through GLUT2 at the basolateral surface) and the intestinal lumen (through reduced SGLT1 and possibly increased apical GLUT2). It also increases GLUT1 expression in the colon and ileum, triggered by the transcription factor ATF4. Once inside the enterocytes, glucose undergoes anaerobic metabolism (fermentation), producing **lactate and acetate** that accumulate in the intestinal wall and are released into the circulation. This intestinal lactate and acetate production establishes a **gut–liver crosstalk** that suppresses the liver's glucose production by two complementary mechanisms: lactate lowers the pH in the portal vein (the vein carrying blood from the gut to the liver), reducing the activity of the hepatic enzyme pyruvate carboxylase; and acetate modifies (acetylates) mitochondrial pyruvate carriers 1 and 2 in the liver, inhibiting their function. Additionally, intestinal lactate production may participate in an "intestinal–liver futile cycle" that increases energy expenditure during long-term metformin treatment, as reported in HFD-fed mice. ### Brown Adipose Tissue: A Temperature-Regulating Target Brown adipose tissue (BAT) — often called "brown fat" — is a highly metabolically active organ best known for its thermogenic role, dissipating energy as heat. A growing number of studies show that BAT also contributes to whole-body glucose regulation, making it a potential therapeutic target for T2DM prevention and treatment. Using 11C-metformin PET imaging, researchers demonstrated metformin uptake in the interscapular (between the shoulder blades) BAT depot of mice, through a transporter called OCT3. The levels of metformin in BAT were similar to those in the kidney and intestine. This finding supports the idea that BAT could be a direct metformin target. In animal studies, metformin treatment increased the expression of cellular proliferation and differentiation markers in brown adipocytes (brown fat cells) alongside increased BAT mass. It also increased the expression of PR domain containing 16 — a brown adipocyte differentiation marker — and rescued BAT mass and function in offspring of obese female mice. Metformin appears to directly target brown adipocyte metabolism, contributing to improved blood lipid profiles through increased clearance of VLDL triglycerides in mice. At the molecular level, metformin promotes intracellular triglyceride breakdown (lipolysis) and mitochondrial fatty acid oxidation in BAT by increasing the activities of hormone-sensitive lipase and AMPK. It also upregulates enzymes involved in fatty acid oxidation and increases the expression of genes related to adaptive thermogenesis in rodents. That said, metformin has only a **limited effect on overall energy expenditure** in most human and animal studies. Researchers have emphasized an intriguing **intestinal AMPK–gut microbiota–BAT axis**: AMPK in the intestine appears to be required for metformin's effects on BAT thermogenic activity by modulating gut bacteria and decreasing circulating levels of a bacterial metabolite called methylglyoxal. Metformin's protective effects on BAT have also been studied in the context of BAT inflammation ("meta-inflammation"), showing that it reduces inflammation by promoting the degradation of HIF1α (a protein that responds to low oxygen levels) as a result of decreased oxygen consumption in macrophages within BAT. This, in turn, restores BAT's responsiveness to cold exposure in obese mice. ## How Metformin Works at the Cellular Level Beyond identifying which organs metformin targets, researchers have been working out which structures inside cells (organelles) the drug interacts with — and the picture has expanded considerably. ### Mitochondria: The Classic Target Since the early 2000s, mitochondria (the energy-producing powerhouses of cells) have been considered the classic target organelles for metformin's glucose-lowering actions. The drug is known to accumulate in the mitochondrial matrix and cause a specific, mild, and reversible inhibition of **mitochondrial respiratory chain complex I** — a critical step in cellular energy production. A landmark study published in **2023** used cryo-electron microscopy (a technique that images frozen molecules at near-atomic resolution) combined with enzyme kinetics to identify **three possible independent binding sites for biguanides on various complex I protein subunits**. The major inhibitory site is located in the amphipathic (part-water-loving, part-fat-loving) region of the quinone-binding channel (Q-channel), adjacent to a mobile structural element in the NDUFS7 subunit. When a biguanide binds to this site, it prevents reactivation of the enzyme's deactivated state. Some rodent studies have suggested alternative mitochondrial mechanisms, such as inhibition of mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) or complex IV. However, the evidence supporting these alternative sites remains highly debated in the scientific community. ### Lysosomes: A Newly Discovered Target The identification of metformin-binding proteins associated with isolated lysosomes (the cell's recycling and waste-processing centers) in human and mouse cells has highlighted lysosomes as an **alternative or additional functional target** of metformin. In primary mouse hepatocytes (liver cells), low concentrations of metformin activate AMPK through a pathway involving a lysosomal complex called the **PEN2–ATP6AP1 axis**. This discovery suggests a new mechanism of action that operates at therapeutic drug concentrations. ### AMPK-Dependent and AMPK-Independent Pathways AMPK is a cellular energy sensor — it detects when energy levels are low and switches the cell into energy-conservation mode. Metformin's mild inhibition of mitochondrial complex I leads to a slight increase in intracellular AMP levels (a sign of energy stress). This rise in AMP triggers two important effects: - **Inhibition of AMP-regulated enzymes** involved in hepatic glucose production, such as fructose-1,6-bisphosphatase and adenylate cyclase, which directly reduces glucose output from the liver. - **Activation of AMPK**, the cellular energy sensor. However, research has shown that AMPK activation itself has **no direct effect on glucose production**. One important debate concerns drug concentrations used in laboratory studies. Many early experiments used suprapharmacological (millimolar) concentrations of metformin — far higher than those achieved in patients. Studies using clinically relevant (micromolar) concentrations show that metformin suppresses glucose production in primary mouse hepatocytes through mechanisms that are **independent of apparent changes in adenine nucleotide levels** (the energy currency molecules). This finding shifts attention toward the low-concentration lysosomal pathway as the more physiologically relevant mechanism. ## Metformin and Your Gut Bacteria Host–gut microbiota interactions are now recognized as a key contributor to metformin's therapeutic effects. The gut acts as a major reservoir for metformin, with drug concentrations far exceeding plasma levels. This creates a unique environment where metformin can directly influence the trillions of bacteria living in the intestine. The gut microbiota, in turn, appears to mediate some of metformin's systemic effects. The proposed intestinal AMPK–gut microbiota–BAT axis suggests that AMPK in the intestine is required for metformin's effects on brown fat thermogenic activity, working by modulating the composition of gut bacteria and reducing circulating levels of the bacterial metabolite methylglyoxal. This complex interplay between drug, host, and microbes represents an exciting frontier in understanding how metformin achieves its full range of beneficial effects. ## Metformin and the Immune System Metformin has well-documented **anti-inflammatory and immunomodulatory properties** in various immune-related diseases. These effects occur through both AMPK-dependent and AMPK-independent mechanisms, involving both the innate immune system (the body's rapid, non-specific defense) and the adaptive immune system (the slower, targeted defense that remembers specific threats). The drug's anti-inflammatory actions have been demonstrated in brown adipose tissue, where metformin alleviates inflammation by promoting HIF1α degradation — a result of reduced oxygen consumption in macrophages (immune cells that engulf and digest debris and pathogens). This, in turn, attenuates macrophage-mediated pro-inflammatory signaling in brown adipocytes and restores the ability of BAT to respond to cold exposure in obese mice. These immune-modulating effects are thought to contribute to metformin's potential benefits in inflammatory diseases, age-related conditions, and even viral infections. ## Metformin, GDF15, and Body Weight Control Elevated circulating levels of **growth differentiation factor 15 (GDF15)** — a stress-responsive cytokine belonging to the transforming growth factor-β family — have drawn attention as a novel biomarker for metformin use. This connection may provide new clues about how metformin produces its beneficial clinical outcomes, particularly with respect to weight loss. In mice fed a high-fat diet, metformin increases the expression of the Gdf15 gene (the gene that codes for GDF15) in the intestine, kidney, skeletal muscle, and liver. Interestingly, the role of AMPK signaling in this gene regulation remains disputed. Human studies have confirmed that GDF15 is released from the gastrointestinal tract in response to metformin. Here is what the research shows about GDF15 and weight: - Increased GDF15 expression and circulating levels are associated with **weight loss and reduced appetite** in humans and mice. - GDF15 works through a specific receptor, **GFRAL** (glial cell-derived neurotrophic factor family receptor-α-like), which is only expressed in the hindbrain (the lower part of the brain). Activation of this receptor facilitates weight loss. - Supporting this pathway's importance, metformin-induced weight loss was **completely blocked in both GDF15-knockout and GFRAL-knockout mice** (mice genetically engineered to lack these proteins). - Metformin promotes body weight loss in people with or without T2DM, and elevated GDF15 levels are associated with metformin-induced weight loss in T2DM patients — likely by reducing food intake, independently of the drug's glucose-lowering effect. However, not all studies agree. Some research challenges this concept by showing that metformin can lower body weight and food intake independently of GDF15–GFRAL signaling in obese mice. Additionally, a study in individuals with prediabetes and overweight who received metformin for up to 13 weeks did **not** confirm a correlation between metformin-induced rises in GDF15 and changes in body weight. These conflicting results raise questions about the importance of long-term drug exposure and individual variability in the weight-lowering response to metformin-induced GDF15 increases — an area that clearly warrants further research. ## Metformin and COVID-19: What Clinical Trials Found Coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), focused intense attention on metformin as a potential protective or therapeutic agent. Advanced age, obesity, and pre-existing T2DM were rapidly identified as risk factors for developing severe COVID-19. This increased risk is believed to be partly linked to chronic low-grade inflammation — known as "inflammaging" (inflammation associated with aging) and/or obesity-associated "metaflammation" (metabolic inflammation). Observational retrospective studies during the first wave of the pandemic suggested that metformin could have both protective and therapeutic roles against COVID-19, despite an increased incidence of lactic acidosis (a potentially dangerous buildup of lactic acid). These potential benefits were thought to arise from metformin's anti-aging, antiviral, and anti-inflammatory properties. However, researchers noted that metformin's apparent benefits might simply reflect its efficient blood sugar control in people with diabetes — and diabetes itself is one of the most important determinants of survival in severe COVID-19. The results of two major clinical trials were decisive: - **TOGETHER trial:** This trial investigated whether early treatment with metformin could lower the risk of emergency care and hospitalization among high-risk patients with COVID-19. However, enrollment was **stopped early** because the intermediate assessment showed metformin had **no effect** on the outcome measures. - **COVID-OUT trial:** This was a phase III, double-blind, randomized, placebo-controlled trial conducted in non-hospitalized adults with obesity or overweight. The trial found that metformin treatment started within 3 days of infection did **not** prevent the occurrence of hypoxaemia (low blood oxygen), emergency department visits, hospitalization, or death. The lack of clinical benefits observed in these rigorous trials did not support repurposing metformin for early treatment of COVID-19. A preprint article (not yet peer-reviewed) suggested metformin might reduce the incidence of long COVID, but this remains to be confirmed. ## Repurposing Metformin: Cancer, Aging, and Inflammation Because of metformin's excellent efficacy and safety record in T2DM, considerable attention has been given to repurposing it for other conditions. The drug has shown promise in several areas: - **Cancer:** Metformin accumulates in tumors at concentrations higher than in plasma. Its effects on cellular energy metabolism and AMPK signaling are being investigated as potential adjunct therapy in cancer treatment. - **Age-related diseases:** Metformin's anti-aging properties — possibly related to its effects on inflammation and cellular metabolism — have made it a candidate for slowing age-related decline and extending healthspan. This is partly related to its effects on "inflammaging." - **Inflammatory diseases:** The drug's anti-inflammatory and immunomodulatory properties, working through both AMPK-dependent and AMPK-independent mechanisms in the innate and adaptive immune systems, suggest potential therapeutic uses in various immune-related conditions. - **Viral infections:** Historically, metformin (called flumamine at the time) was used in **1949** during an influenza outbreak in the Philippines and was noted to lower blood glucose in some patients. In recent years, its use has been explored in patients with T2DM and obesity who contract influenza or COVID-19, though the COVID-19 trial results were negative. ## Clinical Implications: What This Means for Patients These findings have several important implications for patients taking or considering metformin: - **The gut matters more than we thought:** The intestine is not just a passive conduit for metformin — it is a major site of action where the drug reduces glucose absorption, stimulates GLP1 (a hormone that helps regulate blood sugar and appetite), and produces metabolites that communicate with the liver. Patients who experience gastrointestinal side effects may find it helpful to know these effects reflect the drug's actual mechanism of action. - **Weight effects may involve GDF15:** Metformin-induced weight loss appears to be linked to the hormone GDF15, which suppresses appetite and acts on a specific receptor in the hindbrain. However, this effect varies between individuals and may not occur in everyone. - **Pregnancy considerations:** Metformin is increasingly used during pregnancy for gestational diabetes mellitus, polycystic ovary syndrome (PCOS), and T2DM. Because metformin crosses the placenta, the long-term effects of fetal exposure have raised concerns. While in utero exposure was generally considered safe, some studies found a higher incidence of small-for-gestational-age birth weight and an increased risk of childhood obesity. However, a reassuring long-term follow-up study showed **similar growth measurements (anthropometrics) in children exposed or not exposed to metformin in utero**. - **COVID-19:** Based on the TOGETHER and COVID-OUT trials, metformin is **not recommended** for early treatment of COVID-19 to prevent severe outcomes. - **New research directions:** Understanding that metformin works at multiple sites — including lysosomes, at low concentrations relevant to patients — opens the door to developing more targeted therapies with potentially fewer side effects. ## Limitations of the Research Like all scientific reviews, this article has limitations that patients should understand: - **Most mechanistic studies are in animals or cells:** Many of the findings on molecular mechanisms came from preclinical models (mice, rats, minipigs, and isolated cells). While valuable, these findings may not always translate directly to humans. - **Concentration debates:** There is ongoing uncertainty about which metformin concentrations are truly relevant for laboratory studies. Many older studies used suprapharmacological (millimolar) concentrations that far exceed those found in patients (typically micromolar). This has led to questions about whether some proposed mechanisms actually operate in real patients. - **Conflicting evidence:** The role of GDF15 in metformin-induced weight loss is contested, with some studies supporting and others challenging the GDF15–GFRAL requirement. The association between GDF15 increases and weight loss was not confirmed in one study of people with prediabetes and overweight. - **Brown fat findings are mostly preclinical:** Evidence for direct metformin action on brown adipose tissue comes primarily from mouse studies. The drug had limited effects on overall energy expenditure in most human studies. - **COVID-19 trials were specific:** The negative results in the TOGETHER and COVID-OUT trials apply to early outpatient treatment. They do not address other potential uses, such as prevention or treatment of long COVID, which remains unconfirmed. - **One cited study was a preprint:** The authors note that one reference (ref. 56) is a preprint that has not yet been peer-reviewed. ## Recommendations for Patients Based on this review, here is practical guidance for patients: 1. **Continue taking metformin as prescribed** if your doctor has recommended it for T2DM, PCOS, gestational diabetes, or other indications. Its effectiveness in controlling blood sugar, improving cardiovascular outcomes, and reducing mortality is backed by more than 60 years of clinical experience. 1. **Talk to your doctor about gastrointestinal side effects.** These are common and often reflect metformin's legitimate activity in the gut. Your doctor may suggest starting at a low dose and increasing gradually, or switching to an extended-release formulation. 1. **Understand that weight loss from metformin varies.** The GDF15-related weight effects are real for some patients but not universal. Metformin should not be viewed as a primary weight-loss drug. 1. **If you are pregnant or planning to become pregnant**, discuss the benefits and risks of metformin with your healthcare provider. While most evidence suggests it is safe, and long-term follow-up of children exposed in utero is reassuring, individualized care is essential. 1. **Do not use metformin to self-treat COVID-19.** Clinical trials showed no benefit for early outpatient treatment in preventing severe outcomes. 1. **Stay informed about new research.** Repurposing metformin for cancer, inflammatory diseases, and age-related conditions is an active area of investigation. New findings could expand its approved uses in the future. ## Frequently Asked Questions ### How does metformin lower blood sugar? Metformin works in several places, not just the liver. It reduces glucose absorption in the gut, triggers release of a hormone called GLP1, and changes gut bacteria. It also mildly inhibits mitochondrial complex I and activates AMPK in cells. These combined effects lower blood sugar, with the gut playing a larger role than previously thought. ### Does metformin cause weight loss? Metformin can cause modest weight loss in some people. This may be linked to increased levels of a hormone called GDF15, which acts on the brain to reduce appetite. However, not everyone loses weight, and the effect varies. It is not considered a primary weight-loss drug, but it can help some patients with type 2 diabetes. ### Is metformin safe to take during pregnancy? Metformin is sometimes used during pregnancy for gestational diabetes, polycystic ovary syndrome, or type 2 diabetes. It crosses the placenta. Some studies found a higher chance of small-for-gestational-age birth weight and later childhood obesity, but a reassuring long-term follow-up study showed similar growth in children exposed or not. Always discuss with your doctor. ### Can metformin prevent or treat COVID-19? Two large clinical trials, TOGETHER and COVID-OUT, found that metformin did not prevent emergency visits, hospitalization, or death when used as early treatment for COVID-19 in high-risk patients. Therefore, metformin is not recommended for treating COVID-19. A preprint suggested it might reduce long COVID, but that is not yet confirmed. ### What are the common gastrointestinal side effects of metformin? Metformin often causes gastrointestinal side effects like diarrhea, nausea, or stomach upset. These effects happen because the drug acts in the gut, reducing glucose absorption and increasing GLP1. Starting at a low dose and increasing gradually, or using an extended-release form, may help. Talk to your doctor if side effects bother you. ### Can metformin be used for cancer or aging? Research is exploring metformin for cancer, age-related diseases, and inflammatory conditions because it affects cellular energy metabolism and inflammation. Metformin accumulates in tumors at higher levels than in blood, but its role in cancer treatment is still under investigation. No new approved uses have been established yet beyond diabetes and some other conditions. ### How does metformin affect the gut and brown fat? Metformin concentrates in the gut at levels 30 to 300 times higher than in plasma. It reduces glucose absorption and stimulates GLP1. It also reaches brown adipose tissue through a transporter called OCT3, potentially improving fat metabolism. These are important new findings, but many come from animal studies, so more human research is needed. ### Should I get a second opinion about taking metformin for type 2 diabetes, especially if I have side effects or am considering it for weight loss or other conditions? A second opinion can help if you have persistent gastrointestinal side effects from metformin or if you are considering it for weight loss, since effects vary and are not universal. It is also useful if you are pregnant or planning pregnancy, as risks and benefits need individual assessment. Clinical trials show metformin is not effective for early COVID-19 treatment, so a second opinion may clarify appropriate uses. Diagnostic Detectives Network provides independent expert second opinions. ## Source Information **Original Article:** "Metformin: update on mechanisms of action and repurposing potential" by Marc Foretz, Bruno Guigas, and Benoit Viollet. **Publication:** *Nature Reviews Endocrinology* (2023). DOI: https://doi.org/10.1038/s41574-023-00833-4 **Author Affiliations:** Université Paris Cité, CNRS, Inserm, Institut Cochin, Paris, France; and Department of Parasitology, Leiden University Medical Center, Leiden, Netherlands. **Corresponding Author:** Benoit Viollet (benoit.viollet@inserm.fr) **Note:** This patient-friendly article is based on peer-reviewed research. It was written to make the scientific findings accessible to a general audience while preserving all key data, statistics, and conclusions from the original publication. The original article should be consulted for full scientific detail, including all references cited within it. --- Publisher: Diagnostic Detectives Network (https://diagnosticdetectives.com) — independent multi-expert medical second opinions, worldwide, private-pay. Author byline: Anton Titov, MD, PhD. Contact: https://diagnosticdetectives.com/pages/contact Canonical page: https://diagnosticdetectives.com/products/metformin-unlocking-the-secrets-of-the-worlds-most-prescribed-diabetes-medication-and-its-potential-for-treating-other-diseases