A relatively general definition of a drug is any substance, except food, that changes the way the body or mind works. A more comprehensive definition would be any chemical introduced into the body, either by ingestion, vaporization and inhalation (smoking), injecting, or any other method, that causes the body or mind to operate differently than it would without the introduction of the given chemical.
In general this term refers to anything that may be pharmacologically active in a living organism.
Street drugs are not generally recommended when having modifications done! See the entries on alcohol, cigarettes, cocaine, MDMA, heroin, and marijuana. Anesthetics and/or analgesics, on the other hand, may be a little more reasonable. With some of the "heavier" modifications/procedures (or bad luck) antibiotics may also be indicated.
Remember: All of the above listed persons are professionals, and as such are not there to judge you, but to help you! Always put your safety and health first!
Drug use can have a severe physical and mental impact on an individual. People should avoid any drugs that affect the immune system, blood pressure and/or impair the mental abilities of the individual. That said, many cultures have traditionally combined suspension with psychedelic, psychotropic, and "mind expanding" drugs to further their spiritual quests. However, as far as we in the West are concerned, this is highly uncharted territory and extreme care should be taken if you choose to combine these activities.