Background Benzodiazepine use is widespread in older people, although its benefit is uncertain.

Aim To investigate the long-term effect of benzodiazepine use upon dementia risk.

Methods A prospective cohort of men seen on five occasions over 22 years with full medication histories, repeat measures of cognitive function and a clinical diagnosis of dementia.

Results Of 1134 men with complete data, 103 (9.1%) had been taking benzodiazepines regularly at one or more phases. These men showed a marked increased incidence of dementia (OR=3.50, 95% CI 1.57 to 7.79, p=0.002), which persisted despite adjustment for psychological distress and other covariates. Men exposed in earlier phases showed a greater association than more recent exposure, counter to what one would expect if this was due to reverse causation, though we failed to demonstrate a dose–response effect with drug duration.

Conclusion The taking of benzodiazepines is associated with an increased risk of dementia.