For the social side of things, the United Nations website briefly covers the economic and social consequences of drug abuse and illicit trafficking.
For the wilder side of pharmaceuticals, Lycaeum, a site that covers mind-altering chemicals, has an in-depth entheogen section (entheogens being psychoactive substances that are used for religious reasons). Each drug is linked to a copious number of articles covering almost any question you could think of; the Heroin section features links on everything from acupuncture in relationship to Heroin withdrawal to Victorian substance abuse in England. While more traditional psychotropics such as peyote and mushrooms are listed and linked, among its other strange bedfellows, alcohol, caffeine, and chocolate are covered as well.
And if you’re just not sure what to call it, the Street Drug Slang Dictionary probably can tell you.
For characters you’re unsure how to medicate, drugs.com provides drug listings for virtually any disease, as well as a limited medical encyclopedia and medical dictionary.
The Drug Enforcement Administration (DEA) includes information on drug policy on its website, covering controlled substances law, penalties, and drug scheduling.
Erowid takes a similar approach to psychoactives, but with a much prettier layout.
While involving only the more legal side of things, the U.S. Department of Health and Human Services covers a wide variety of drug- and food-related topics in its Drug & Food Information section, including some links to biotechnology as well as simpler information on immunizations, water safety, and prescriptions drugs.