What are the side effects of metformin?
Metformin causes gastrointestinal complaints such as diarrhoea, nausea and vomiting in a substantial proportion of users. Discuss with your doctor whether the dose can be built up gradually, or consider an alternative if the complaints persist.
Gastrointestinal complaints are the most common side effects of metformin. A randomised study in women with PCOS found that 31.6% experienced diarrhoea, 14.0% vomiting, 13.2% nausea, 11.4% loss of appetite and 8.8% stomach pain. Compared with placebo or acupuncture, these complaints occurred significantly more often.
How large the overall risk of side effects is was examined in a Cochrane review of women using metformin around the time of IVF. The metformin group had more than three times the chance of experiencing side effects compared with the placebo group. The researchers rated the certainty of this figure as low, but the direction is clear: metformin substantially increases the risk of side effects.
A rarer but more serious side effect is lactic acidosis: a build-up of lactic acid in the blood that can be life-threatening. The likelihood of this is small, but the risk does exist. People with poor kidney function are at higher risk, because metformin is then excreted less efficiently.
The gastrointestinal complaints can be partly reduced by taking metformin with a meal or by increasing the dose gradually. Nevertheless, the side effects are reason enough for some people to stop or switch to an alternative, which also explains why researchers are looking for agents with a comparable effect but fewer complaints.
All claims are based on studies in women with PCOS (RCTs and a Cochrane review). Whether the frequency of side effects is comparable in other groups (such as people with type 2 diabetes) cannot be determined from the available sources.