In the present patient population, patients younger than 10 years of age, all of whom had intellectual disability, were more likely to have side-effects than those older than 10 years of age. Observed adverse effects, which were generally mild, occurred in patients with baseline intellectual disability, attention deficit disorder and behavioural problems. Behavioural adverse effects warranted discontinuation of the medication in only three patients. The severity of intellectual disability (mild versus moderate or severe) did not affect the extent of the response or the occurrence of side-effects.