Abstract: The invention provides a pharmaceutical composition for oral administration of a pharmaceutically active agent to a subject, including the pharmaceutically active agent and an inhibitor of CYP3A4. Administration of the inhibitor and the pharmaceutically active agent reduces pre-systemic degradation of the pharmaceutically active agent by CYP3A4. The inhibitor can be poly(ethylene glycol), methoxy poly(ethylene glycol), aminated poly(ethylene glycol), O-(2-aminoethyl)-O-methoxy poly(ethylene glycol), polyoxyethylene glycol, branched poly(ethylene glycol), 3-arm poly(ethylene glycol), 4-arm poly(ethylene glycol), 8-arm-poly(ethylene glycol)polyamine, poly(L-lysine), poly(L-arginine), poly(L-alanine), poly(L-valine), poly(L-serine), poly(L-histidine), poly(L-isoleucine), poly(L-leucine), poly(L-glutamic acid), poly(L-glutamine), poly(L-guanidine), poly(methyl methacrylate), polyvinyl acetate, polyacrylate, poly(lactic-co-glycolic acid) and derivatives thereof. A method of treatment is also described.