This study from Yale University School of Medicine in Connecticut looked at 426 hospitalised patients, aged 70 years and older, 114 of whom had received diphenhydramine. Those taking the drug had a 1.7 times greater risk of delirium compared with those not taking the drug, and the greater the dose, the greater the risk. These patients also had around twice the risk of disruption of their sleep-wake cycle and abnormal psychomotor activity, three times greater risk of inattention and altered consciousness, and more than five times the risk of disorganised speech and behavioural disturbance.

In addition to delirium, patients using diphenhydramine tend to develop bladder problems calling for catheterisation, and require a significantly longer stay in hospital (Arch Intern Med, 2001; 161: 2091-7).