Xenon Pharmaceuticals has voluntarily paused new patient enrollment in its clinical trials testing azetukalner for major and bipolar depression, citing neuropsychiatric and nervous system-related side effects. The decision came after the company consulted its safety monitoring board and reviewed adverse event data across the studies. Patients already enrolled will continue their treatment without interruption.

The side effects observed included confusion, speech difficulties, and motor coordination problems — and a small number of cases were classified under the broad category of "psychosis." Company executives noted the events were short-lived and reversible, with no long-term negative effects reported. Xenon is now evaluating potential dose adjustments to improve tolerability.

Key Takeaways:

• The psychosis rate was approximately 1% or under, described as brief and fully reversible
• Side effects were consistent with the drug's mechanism of action but had not appeared in an earlier mid-stage depression study — likely because that trial enrolled far fewer patients
• The pause does not affect Xenon's epilepsy program; the company has already submitted an FDA application for azetukalner to treat focal seizures
• Xenon shares fell more than 25% in after-hours trading following the announcement

Why it matters: Azetukalner was seen as a promising candidate in the depression treatment pipeline. This pause raises important questions about the drug's safety profile at scale and could delay its path to approval for psychiatric indications — a setback for patients with limited treatment options.