A Common Drug Causing a Common Side Effect at an Uncommon Time: Metformin-Induced Chronic Diarrhea and Weight Loss After Years of Treatment

Type 2 diabetes is a common condition, with a worldwide prevalence of 9.3% as of 2019. Metformin is the most common drug used and is the first-line agent in type 2 diabetes management. It is also considered one of the safest pharmacological agents for the treatment of the disease. However, gastrointestinal side effects are common, occurring in up to 75% of those who take metformin. These side effects may include diarrhea, nausea, vomiting, abdominal discomfort, and/or flatulence, with severity ranging from mild to severe. These complications are more common with immediate-release preparations of metformin than with its extended-release formulation. Discontinuation and nonadherence rates have been as high as 46% with metformin because of these issues. Here is a case series of patients with both adverse side effects--diarrhea and weight loss--that began after long-term use of metformin and a review of the related literature.