PURPOSE: The aim of this study was to gather information about frequency, latency time, outcome and management of frequently occurring adverse drug reactions (ADRs) related to the use of metformin in daily practice. METHODS: A prospective, observational cohort study was performed. A total of 2490 first-time metformin users were recruited through pharmacies in the Netherlands between February 1, 2008, and April 1, 2012. Patients were invited to complete six web-based questionnaires at 2-week, 6-week, 3-month, 6-month, 9-month and 12-month intervals after starting treatment with metformin. Information was gathered about patient characteristics, ADRs and drug use. RESULTS: The occurrence of at least one possible ADR related to the use of metformin was reported by 34.5 % of the patients. A higher proportion of females reported the occurrence of an ADR (39.6 %) compared to the proportion in males (30.9 %). Some patients (11.4 %) stopped using metformin within 1 year after start. More than half of the patients (50.8 %) undertook no action regarding metformin after the occurrence of ADRs. A high number of patients (77.7 %) recovered or were still recovering from ADRs despite continuation of metformin. Most ADRs occurred shortly after the beginning of the treatment, with a median latency time of 1-6 days. The study revealed some ADR-specific differences in occurrence rate, latency time, management and outcome. CONCLUSION: This study successfully obtained information about frequency, latency time, outcome and management of frequently occurring ADRs related to the use of metformin in daily practise.