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Blood sugar and body weight spike fast after starting second-generation antipsychotics — and the first three months are the danger zone. A large UK study found A1c rises ~14x faster and weight climbs ~9x faster in the first 90 days of treatment. Crucially, weight gain alone doesn't tell the whole story, meaning clinicians need to monitor both metrics early and often.
Starting a second-generation antipsychotic (SGA) triggers a rapid metabolic response — and new research published in The Lancet Psychiatry shows just how fast things can change. In a study of nearly 97,000 adults with severe mental illness in England, A1c levels rose about 14 times faster and body weight increased about 9 times faster during the first 90 days of SGA treatment compared to the pre-treatment period.
What makes this especially important: weight and blood sugar don't always move together. Quetiapine, for example, caused significant weight gain but minimal A1c changes, while olanzapine drove steep increases in both. Women — particularly those on olanzapine — faced the greatest early metabolic burden. After 90 days, the rate of change slowed considerably, but neither measure returned to baseline.
By the Numbers:
Why it matters: Clinicians often rely on weight as a proxy for metabolic risk — but this study shows that's not enough. Experts recommend checking both weight and A1c at baseline and again at 12 weeks, and considering early metformin use, especially for patients starting olanzapine or clozapine.
