Pharmacokinetics and drug metabolism play an important role as determinants of in vivo drug action. The CYP450 enzyme family plays a determinant role in the biotransformation of a vast number of structurally diverse drugs. Many drug interactions are a result of the inhibition or induction of CYP enzymes. The non-compartmental pharmacokinetic analysis is the most used method for analyzing data from a drug interaction study. Compartmental analysis can be also useful and sometimes more informative than non-compartmental analysis. Many efforts to reduce polypharmacy are important, and pharmacokinetic tools used to study the mechanism of drug-drug interactions may help in a better management of pharmacotherapy including the avoidance of clinically relevant drug interactions.