The protease inhibitors are a new class of drugs for the treatment of HIV infection. Results of treatment have proved beneficial to HIV positive patients, resulting in slower disease progression to aids and death. However the potential of drug interactions is high because of the drug-transporting P-glycoprotein and cytochrome P450 3A4-mediated metabolism. Administration of protease inhibitors may result in increased or decreased concentrations of co-administered drugs, and the plasma concentration of protease inhibitors may be affected by other drugs. It is possible to take advantage of the interactions by combining two protease inhibitors. Attention is drawn to the protease inhibitors and their possible interactions because of the advantages and disadvantages this implies. It is possible to monitor interactions by measuring plasma concentrations of the protease inhibitors.