Many drugs, both legal and illegal, have side effects which impact on the user's sexual functions. For example, the side effect of many legal antidepressants and antipsychotic drugs is the reduction of sexual desire.
Other drugs, such as alcohol, are notorious for being used to render unknowing victims unconscious or severely sedated and thus easy targets for sexual predators.
Diphenhydramine or as the pharmaceutical name Benadryl can powerfully enhance sexual arousal and lead to an extremely powerful orgasm in higher than recommended dosages but also could decrease sexual arousal and desire depending on the situation.
Perhaps the most common drug used is alcohol. At low concentrations of blood alcohol, social inhibitions are reduced, though in higher concentrations it can also inhibit performance. Many other drugs also inhibit sexual performance.
Tobacco use (e.g., cigarette smoking), also reduces sexual function, with the incidence of impotence being approximately 85 percent higher in male smokers compared to non-smokers.
A few drugs can actually increase sexual performance when used to treat erectile dysfunction. These include sildenafil (marketed as Viagra) and tadalafil. Bremelanotide appears to affect sexual desire directly, making it the first scientifically recognized aphrodisiac. This is also true of Melanotan II which bremelanotide is based upon. Additionally, the alkyl nitrites (poppers) have a long history of use as a sexual enhancement aid, going back about fifty years. According to the text "ISOBUTYL NITRITE and Related Compounds", many researchers agree that the alkyl nitrites may be a true aphrodisiac in the sense of promoting and enhancing sexual response. 
Kane Race (2009) Pleasure Consuming Medicine: The Queer Politics of Drugs. Duke University Press.