Interactions with antiepileptic drugs are common and may have important clinical consequences. The physician should always consider carefully the need for and the implications of adding a new drug to any therapeutic regime and should be prepared to think about the possibility of an interaction whenever an unusual response is seen. Serum drug levels can be an invaluable tool in the recognition and the management of most types of interactions, but in no case can they be a substitute for careful observations and evaluation of the patient's symptoms and physical findings.