Twenty-six children with intellectual disability and six normal children, all 
suffering from refractory partial seizures, received open-label gabapentin 
(range = 10-50 mg kg(-1) day(-1); mean = 26.7 mg kg(-1) day(-1) as an add-on 
medication to their antiepileptic drug regimen. Mean seizure frequency during 
baseline was 9.5 seizures per week. Both groups had a significant reduction in 
seizure frequency. Response scores and response ratios did not differ between 
the intellectually disabled and normal groups (1.67+/-0.67 and 1.25+/-0.69, P = 
0.697, and -0.400+/-0.089 and -0.283+/-0.159, P = 0.961, respectively). 
Behavioural side-effects were more likely to occur in patients with intellectual 
disability in comparison with the mentally normal group (P = 0.0107). 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. It is concluded that 
gabapentin is equally effective as an add-on medication against partial seizures 
in patients with or without intellectual disability. However, children with 
intellectual disability who also are less than 10 years of age with baseline 
attention deficit appear to be at a higher risk of behavioural side-effects.