Drug interactions can alter the pharmacokinetics and/or pharmacodynamics of a drug. In pharmacokinetic drug interactions, the concentrations of 1 or more drugs are altered by another. This change in concentration in a given drug may be due to changes in absorption, distribution, metabolism, or elimination. The pharmacodynamic interaction can lead to additive, synergistic, or antagonistic effects of a drug. Drug interaction studies are regularly conducted with conventional drugs (small molecules), but very few drug interaction studies have been performed with macromolecules (therapeutic proteins or monoclonal antibodies). This is mainly because most macromolecules are not metabolized by the cytochrome P450 system, and their mechanism of elimination is complex. However, it has been shown in several studies that interferons can have an impact on the cytochrome P450 system that may alter the pharmacokinetics and pharmacodynamics of a conventional drug when given with interferons. Therefore, it is important to evaluate the effect of other classes of macromolecules (cytokines, interleukins, monoclonal antibodies) on drug-metabolizing enzymes. It is also imperative that the effects of conventional drugs on the pharmacokinetics and pharmacodynamics of macromolecules be conducted. The present review encompasses several drug interaction studies that were conducted with macromolecules and highlights the impact of these studies on the pharmacokinetics and/or pharmacodynamics of the involved drugs.