Metformin-Associated Gastrointestinal Intolerance: A Narrative Review of Mechanisms and Clinical Management

Metformin-associated gastrointestinal (GI) intolerance is a frequent clinical problem that can limit treatment initiation, delay dose escalation, and reduce long-term adherence in patients with type 2 diabetes mellitus. Common symptoms include nausea, diarrhea, abdominal discomfort, and bloating, although symptom pattern and severity vary substantially between individuals. This narrative review summarizes current evidence on the determinants, mechanisms, and clinical evaluation of metformin-associated GI intolerance. In routine practice, assessment should begin with potentially modifiable exposure-related factors, including dose, single-dose burden, titration pace, formulation, administration with meals, kidney function, and concomitant medications. If symptoms persist or appear disproportionate to treatment exposure, clinicians should then consider broader host susceptibility, including baseline GI vulnerability, microbiome-related influences, altered bile acid handling, mucosal and neuroregulatory responses, comorbidity burden, and polypharmacy. This exposure–susceptibility framework provides a practical way to interpret metformin-related GI symptoms in routine care. It supports a stepwise clinical approach in which modifiable contributors are addressed first, broader context is reviewed when needed, and premature discontinuation is avoided whenever possible. Despite limitations in the current evidence base, available data support a structured and clinically useful approach to metformin intolerance.