Modifications in the effect of a drug are caused by differences in the absorption, transport, distribution, metabolization or excretion of one or both of the drugs compared with the expected behavior of each drug when taken individually. These changes are basically, the modifications in the concentration of the drugs. Inhibition: Fluvoxamine inhibits CYP 1A2 and the serum level of theophylline increases. Induction: Carbamazepine and phenobarbital induce cytochrome P450 enzymes and eventually the metabolism of TSA increases. Treatment of ADHD in case of epilepsy comorbidity: Dexamphetamine may be preferred to methylphenidate for children with both ADHD and epilepsy. Although children with ADHD are increasingly recognized as being at an elevated risk for seizures, treatment of ADHD symptoms with atomoxetine does not appear to elevate this risk further. The shared vulnerability between ADHD and seizure activity should be taken into account when making treatment decisions for populations of children with epilepsy and children with ADHD. No interactions with any of the AEDs have been reported for atomoxetine, while MPH can increase phenytoin serum concentrations. Furthermore, MPH serum concentrations may be lowered by the contemporary administration of carbamazepine, thus leading to a loss of efficacy.