The invention provides methods of reducing the side effects of tramadol. Accordingly, in one embodiment, the invention provides a method of reducing the incidence of newly-discovered side effects related to sexual function in human males taking a tramadol material. The method comprises administering a phosphodiesterase inhibitor to a male taking the tramadol material. The invention also provides pharmaceutical compositions. In one embodiment, the composition comprises a tramadol material and a phosphodiesterase inhibitor. The invention further provides kits. In one embodiment, the kit comprises a tramadol material and a phosphodiesterase inhibitor.