Psychoactive drugs are chemical substances that alter perceptions, mood, consciousness, or behavior by acting on the nervous system. They include legal substances such as caffeine, alcohol, and nicotine as well as controlled or illicit compounds.

Different classes produce different effects. Depressants slow neural activity and body functions. Stimulants increase alertness and energy. Hallucinogens distort sensory experience and can produce profound changes in the sense of self and reality. Opioids relieve pain and produce euphoria while carrying high risk of dependence. Most psychoactive drugs work by enhancing, blocking, or mimicking the action of neurotransmitters.

Tolerance develops with repeated use, so larger doses are needed for the same effect. Dependence can be physical, psychological, or both. Withdrawal symptoms appear when use stops. Risk of harm varies widely with the substance, dose, route of administration, and individual vulnerability. Public-health approaches emphasize education, regulation, and treatment rather than solely punitive measures.

Any chemical that reliably changes the way the mind works belongs in the broad category of psychoactive drugs.