Based on NIH | Does metformin cause diarrhea, and if so, how common is it and what strategies can help manage it?

Metformin and diarrhea: what to expect
Metformin can cause diarrhea for some users, and this is among its most common side effects. ^[1] In clinical trials of the extended‑release (XR) form used alone, diarrhea occurred in about 9–10% of people taking metformin XR versus about 2–3% on placebo, showing it happens more often than with no treatment. ^{[2] [3]} Diarrhea is a leading reason some people stop metformin, though in XR studies only a small percentage discontinued because of it (around 0.6–1%). ^{[4] [3]} In real‑world clinic surveys, metformin users reported higher rates of diarrhea than those not taking metformin (about 20% vs 6%), and symptoms usually resolved within a few days after stopping the drug. ^[5]

How common is diarrhea with different formulations?

• Immediate‑release (IR) metformin: Diarrhea is common and is one of the most frequent reasons for stopping therapy; discontinuation due to diarrhea has been reported around 6% with IR tablets in some labeling summaries. ^[6]
• Extended‑release (XR) metformin: In placebo‑controlled trials, diarrhea occurred in roughly 9.6–10% on XR vs 2.6–3% on placebo, and discontinuation due to diarrhea was uncommon (about 0.6–1%). ^{[2] [3] [4]}
• Observational data suggest XR may be better tolerated than IR for gastrointestinal (GI) side effects, which can help adherence. ^[7]

Why diarrhea happens

The exact cause isn’t fully settled, but metformin can affect gut processes such as bile‑salt handling, serotonin signaling, and local glucose metabolism in the intestines, which may lead to loose stools, gas, or nausea in susceptible individuals. ^[8]

Practical strategies to prevent and manage diarrhea

• Start low, go slow: Taking a low dose and increasing gradually often helps the gut adjust and can lessen diarrhea. ^[1]
• Take with meals: Swallowing metformin with food can reduce stomach upset and diarrhea for many people. ^[1]
• Consider XR formulation: If diarrhea persists on IR tablets, switching to extended‑release may reduce GI side effects while maintaining glucose control. ^[7]
• Temporary dose reduction or pause: If symptoms are significant, clinicians may suggest lowering the dose or briefly stopping, then re‑titrating once symptoms improve. ^{[9] [10]}
• Monitor duration: Mild GI symptoms often improve after a while; if diarrhea lasts beyond a few weeks, recurs after settling, or starts later in therapy, it’s reasonable to contact a clinician to reassess dosing or formulation. ^{[9] [10]}
• Rule out other causes: Other medicines, supplements, or infections can also cause diarrhea; a clinical review may be needed if symptoms are severe, persistent, or accompanied by fever, blood, or signs of dehydration. ^[11]

When to seek help

If diarrhea is frequent, causes dehydration, weight loss, nighttime incontinence, or does not improve with the steps above, it may be worth discussing an alternative formulation or therapy with your healthcare professional. ^[9] Severe or persistent symptoms warrant evaluation to exclude other causes and to ensure safe blood sugar management. ^[10]

Key takeaways

• Diarrhea is a recognized and relatively common side effect of metformin, particularly early in treatment, and occurs more often than with placebo. ^{[2] [3]}
• Many people find symptoms lessen over time, especially when taking doses with food or using the extended‑release formulation. ^[1]
• Only a small proportion need to stop metformin XR due to diarrhea in clinical studies, but individual experiences vary. ^{[4] [3]}
• Thoughtful dose titration, taking with meals, and switching to XR are practical, evidence‑based approaches to improve tolerance. ^{[1] [7]}