Methods of inhibiting cytochrome P450 2D6 enzymes are provided that can be used for improving the treatment of diseases by preventing degradation of drugs or other molecules by cytochrome P450 2D6. Pharmaceutical compositions are provided that can act as boosters to improve the pharmacokinetics, enhance the bioavailability, and enhance the therapeutic effect of drugs that undergo in vivo degradation by cytochrome P450 2D6 enzymes.