Cytochrome P450 oxidoreductase (POR) has played a potential role in the metabolism of drugs and steroids by supplying electrons to microsomal cytochrome P450 (CYP) enzymes. More than 200 different POR mutations and polymorphisms causing more than 130 amino acid changes in the POR protein have been reported since 2004. A503V is a common amino acid sequence variant encoded by POR*28, whereas A287P and R457H are the most common disease-causing mutations in Europeans and Asians, respectively. Polymorphisms in the POR gene can affect POR activity, CYP-mediated drug metabolism activities, and the efficacy of several clinically used drugs. The effects of POR variants on CYP activities are substrate dependent. In this review, recent research on the effects of POR genetic polymorphisms on drug metabolism and therapy has been summarized and discussed, which can contribute to the rational use of drugs in clinic and the development of personalized medicine.