Substance Misuse - General Practice: The Integrative Approach. We define a drug (substance) as a chemical entity, self administered (non-medically) for its psychoactive effect. The effect usually includes a change in mood, arousal or perception, thinking (cognition) and/or behaviour. It may vary according to which drug is used, the amount used, the route of administration, the mixture of drugs used, the expectations of effect by the user, the setting of use and the personal characteristics of the user such as weight, gender and previous drug experience. Drugs can be classified according to their physiological effects: depressants, opioids, cannabinoids, stimulants and hallucinogens. In this chapter we briefly consider the more important drugs in most of the categories, and for each we consider the drug and its mode of action, use and clinical effects, and current medical and integrative treatments.