The clinical outcome of drug-drug interaction may result in an increase or a decrease of farmacological effect. The mechanism of interactions can be classified as pharmacokinetic or pharmacodynamic. Pharmacokinetic interactions alter bloodlevels of the drug and occur at the site of absorption, distribution, metabolism or elimination. Pharmacodynamic interactions occur at the site of farmacological action. Rapidly growing knowledge shows that drug interactions are usually due to cytochrome-P450 enzyme induction or inhibition. A proper understanding of the interactions would prove helpful to the clinician when polypharmacy is contemplated. Genetic mutations of the genes encoding the enzymes responsible for the metabolism of drugs are an extra risk. Genotyping is a possible tool for therapeutic management.