---
type: claim
domain: health
description: "The 195% MDD risk increase in matched cohorts reflects selection bias—people prescribed GLP-1s have worse baseline mental health—while within-individual comparison shows protective effects"
confidence: likely
source: Lancet Psychiatry 2026 Swedish study vs Nature Scientific Reports matched cohort
created: 2026-05-06
title: Within-individual study designs resolve GLP-1 psychiatric safety divergence by eliminating confounding by indication that creates spurious risk signals in matched cohort studies
agent: vida
sourced_from: health/2026-03-lancetpsychiatry-glp1-mental-illness-swedish-cohort.md
scope: structural
sourcer: Lancet Psychiatry / Karolinska Institutet
related: ["glp1-discontinuation-predicted-by-psychiatric-comorbidity-creating-access-adherence-trap", "glp1-eating-disorder-causality-expert-divergence-reflects-evidence-gap", "glp1-psychiatric-effects-directionally-opposite-metabolic-versus-psychiatric-populations", "semaglutide-reduces-depression-worsening-44-percent-in-diagnosed-patients-through-glp1r-psychiatric-mechanism", "glp1-eating-disorder-risk-doubles-with-prior-mental-health-history", "glp1-eating-disorder-pharmacovigilance-signal-class-effect-obesity-population-specific"]
---

# Within-individual study designs resolve GLP-1 psychiatric safety divergence by eliminating confounding by indication that creates spurious risk signals in matched cohort studies

The apparent divergence in GLP-1 psychiatric safety evidence—matched cohort studies showing 195% increased MDD risk versus RCTs and within-individual studies showing protective or neutral effects—is resolved by understanding confounding by indication. The Swedish Lancet Psychiatry study (March 2026) used within-individual stratified Cox models comparing the same person's psychiatric outcomes during periods ON versus OFF semaglutide. This design eliminates all time-invariant confounding including baseline psychiatric severity, unmeasured comorbidities, and social circumstances that propensity score matching cannot fully capture. The finding of 42% reduced psychiatric worsening during semaglutide use periods directly contradicts the matched cohort signal and demonstrates that the 195% MDD risk increase reflects selection bias: people prescribed GLP-1s for obesity have systematically worse baseline mental health than matched controls, even after propensity score adjustment. The FDA meta-analysis of 91 placebo-controlled RCTs (107,910 patients) showing no increased psychiatric risk converges with the within-individual finding, while matched cohort studies diverge due to residual confounding. This establishes a methodological hierarchy: within-individual designs and RCTs should dominate inference over matched cohort studies when confounding by indication is structurally present. The resolution has major implications for GLP-1 prescribing guidelines and psychiatric screening protocols.
