Starting antipsychotics linked to early blood sugar and weight increases

Clinical takeaway: Check A1c and weight before starting a second-generation antipsychotic and reassess both at 12 weeks. Use baseline risk and early changes to guide closer follow-up, lifestyle support, and medication review.

People with severe mental illness already face excess cardiovascular and diabetes risk, which antipsychotic treatment can compound. A large English study identifies the first three months after treatment initiation as a critical window for detecting metabolic changes.

Researchers analyzed primary care records from 96,849 adults aged 18–59 years with severe mental illness and no preexisting type 2 diabetes. They compared A1c and weight trajectories before and after the first recorded oral second-generation antipsychotic prescription.

Both measures increased much faster during the first 90 days. Estimated average weight gain was approximately 2.7 kg during that period. Afterward, increases slowed toward pretreatment rates rather than reversing the early increases.

Olanzapine users and women generally had the steepest early increases. Weight and A1c did not consistently move together. Quetiapine was associated with substantial weight gain but little early A1c change. The findings reinforce that weight alone cannot reliably indicate glycemic risk.

Baseline prediabetes identified a particularly vulnerable group. Estimated two-year diabetes incidence was 19% among participants with prediabetes versus 3% among those with normal baseline A1c, using the study’s UK glycemic definitions.

“By checking blood sugar and weight early, doctors may be able to spot problems sooner and offer support, advice, or treatment to help prevent more serious health issues later on,” said lead author Juan Carlos Bazo-Alvarez, PhD, of University College London. “This could mean that patients are able to receive the benefits of antipsychotic medicines while reducing their risk of developing conditions such as type 2 diabetes.”

Source: Bazo-Alvarez JC, et al. (2026 Oct 14) Lancet Psychiatry. Change in HbA1c and weight after initiation of antipsychotic treatment in people with severe mental illness in England: an individual-level interrupted time-series analysis