Semaglutide and weight in schizophrenia

Clozapine and olanzapine are among the most effective antipsychotic medicines available for schizophrenia, but they carry a well-documented metabolic cost: substantial weight gain that shortens life expectancy and raises cardiovascular risk. A new open-label trial published in Schizophrenia Research (PMID 42372344) ^[1] is the first to report body-weight, waist-circumference, and gut-microbiome data together from a dedicated semaglutide intervention in this specific population.

This article summarises what the study found, what its limitations are, and what it does not change about how antipsychotic treatment decisions should be made. This is not medical advice. Any person taking clozapine or olanzapine who is considering adding a weight-loss medicine must discuss that decision with their treating psychiatrist before making any changes.

What the study did

The trial enrolled 26 adults (mean age 41.5 years; 65.4% female) with a confirmed schizophrenia diagnosis, a BMI above 27 kg/m², and current treatment with clozapine or olanzapine. Importantly, participants did not have diabetes. Semaglutide was given over 24 weeks: an eight-week dose-escalation phase followed by 16 weeks at full dose. Participants were then followed for a further 76 weeks after the intervention ended. The design was open-label, meaning participants and researchers knew what was being administered.

Seventeen of the 26 participants completed the 24-week intervention (65.4% completion). Of those, 15 of 17 (88.2%) remained in the longer follow-up period. The relatively small sample and open-label design are important cautions when reading the results.

The weight loss numbers

On an intent-to-treat basis, semaglutide was associated with a mean body-weight reduction of 9.8% (approximately 10.1 kg) ^[1] over the 24-week intervention. Waist circumference fell by 7.3%. HbA1c (a marker of blood-sugar regulation) fell by 5.3%, though that result did not reach conventional statistical significance (p=0.055).

At the end of the 76-week follow-up period -- after semaglutide had been stopped -- the average weight change from baseline had attenuated to a loss of 5.1% (about 5.3 kg). The trajectory is consistent with what has been observed in the general GLP-1 class literature: weight loss is largely tied to continued use, and body weight tends to partially rebound after discontinuation. The SURMOUNT-4 trial of tirzepatide demonstrated the same pattern in a non-psychiatric population.

The gut microbiome signal

A distinctive feature of this trial is that it measured gut microbial composition alongside weight outcomes. As time on semaglutide increased, alpha diversity (a measure of how varied the gut microbial community is) decreased. The species Parasutterella excrementihominis became enriched during the intervention.

The clinical meaning of these microbiome changes is not yet clear. The authors describe the shifts as "consistent with improvement in health outcomes," but the study was not designed or powered to test whether microbiome change drove weight loss, followed from it, or was incidental. Gut microbiome science in the context of GLP-1 medicines remains an early-stage field, and single-arm open-label data in 26 participants cannot establish causal direction.

Why weight matters so much in this population

People living with schizophrenia have a life expectancy roughly 15 to 20 years shorter than the general population, with cardiovascular and metabolic disease accounting for much of that gap. Clozapine, the most efficacious antipsychotic for treatment-resistant schizophrenia, is strongly associated with weight gain and metabolic dysfunction. Olanzapine carries a similar profile. Earlier research -- including a liraglutide randomised controlled trial (PMID 28601891) and a semaglutide case series (PMID 37113745) ^[2] -- suggested GLP-1 receptor agonists could help counter antipsychotic-induced weight gain, but data specific to semaglutide in a dedicated schizophrenia trial have been limited. A concurrent placebo-controlled Phase 2 trial (the COaST trial, PMID 40506208) is addressing that evidence gap with a randomised design.

What the study cannot tell us

Several limitations are worth naming directly. The sample was small (n=26) and there was no placebo or comparator arm, so weight loss cannot be attributed to semaglutide alone with certainty. The open-label design means participant behaviour may have changed in ways that contributed to the result. The 76-week follow-up describes weight trajectory after stopping but does not address safety or efficacy of long-term continued use. The study also did not enroll people with diabetes, so findings do not extend to that common comorbidity in this population.

The antipsychotic medications themselves were not changed during the trial. This is a critical point: the study tested an add-on intervention, not an antipsychotic switch. Clozapine and olanzapine efficacy for schizophrenia was maintained throughout.

What this does not change

The results of this trial do not constitute a practice guideline, a prescribing recommendation, or grounds for changing any individual's psychiatric medication regimen. Schizophrenia treatment is one of the highest-stakes clinical areas in medicine. Adding, stopping, or adjusting any medicine -- including semaglutide -- in a person taking an antipsychotic requires psychiatric oversight. The interaction profile between semaglutide and clozapine or olanzapine, the effect of weight loss on clozapine plasma levels (which are weight-sensitive), and individual metabolic status all require clinical judgement that a study summary cannot replace.

Anyone taking clozapine or olanzapine who is concerned about weight should raise it with their psychiatrist at their next scheduled appointment, not act on research findings independently.

The broader research picture

This trial sits alongside a small but growing body of evidence testing GLP-1 receptor agonists in people with serious mental illness. The SemaPsychiatry randomised trial protocol (PMID 36720576) and the COaST Phase 2 RCT (PMID 40506208) are both evaluating semaglutide with more rigorous designs. A 2024 systematic review and meta-analysis (PMID 42238556) concluded that semaglutide has promising efficacy data for antipsychotic-induced weight gain but noted the evidence base remains thin. The Lappin et al. (2026) open-label data add a microbiome dimension and longer follow-up, but controlled trial evidence will be needed before clinical practice shifts. For background on how semaglutide works as a GLP-1 receptor agonist, see the semaglutide overview.