Weight gain is one of the most distressing and harmful side effects of antipsychotics, yet preventive strategies remain underused. This international panel developed the first evidence-based guideline for using metformin to prevent antipsychotic-induced weight gain (AIWG).
Across randomized trials and cohort studies, metformin reduced weight gain by about 3–4 kg when started at the same time as antipsychotics, with additional benefits on glucose and cholesterol. Side effects were generally mild and transient (mostly GI upset).
The panel strongly recommended starting metformin in these situations:
With high-risk antipsychotics (olanzapine, clozapine).
With medium-risk antipsychotics (quetiapine, risperidone, paliperidone) in youth or in patients who have cardiometabolic risks.
With any antipsychotic if >3% weight gain within the first year of treatment.
Lifestyle counseling remains essential, but metformin offers a safe, inexpensive, and effective pharmacologic strategy.
CARLAT TAKE This guideline moves metformin into the mainstream for preventing AIWG. For patients starting olanzapine or clozapine—especially younger ones—consider co-prescribing metformin from day one.
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