Background: Aggression is a major concern in psychiatric in-patient care. Nutritional supplementation has been found to reduce aggressive incidents in forensic populations and children with behavioural problems.

Aims: To assess whether multivitamin, mineral and n-3 polyunsaturated fatty acid supplementation would reduce the number of aggressive incidents among long-stay psychiatric in-patients.

Method: Data were collected at eight local sites for mental healthcare in The Netherlands and Belgium. Participants were randomised to receive 6-month treatment with either three supplements or placebo. The primary outcome was the number of aggressive incidents, determined by the Staff Observation Aggression Scale – Revised (SOAS-R).

Results: In total, 176 participants were randomised. The primary outcome of SOAS-R incidents was similar in supplement and placebo groups, with a rate ratio of 1.08 (95% CI 0.67–1.74, P = 0.75).

Conclusions: Six months of nutritional supplementation did not reduce aggressive incidents among long-stay psychiatric in-patients.