What is a psychoactive substance?  

A psychoactive substance is any drug or food that affects mental ability, activity or processes, or mood. Psychoactive substances range from ordinary everyday substances and over-the-counter drugs such as caffeine, alcohol, chocolate and antihistamines, to strong illegal drugs such as heroin. Psychoactive substances can lift your mood or make you depressed; they can make you feel more awake or lull you into a stupor. Some psychoactive substances, such as LSD, can cause altered states of consciousness and hallucinations. Certain drugs in the opium family, such as heroin, oxycodone and hydrocodone, cause temporary euphoria. Stimulant drugs, such as Adderall and methamphetamine, can make you feel powerful and alert.

Alcohol is a psychoactive substance?

Alcohol is a psychoactive substance that can cause temporary personality changes and it affects the ability to think and problem-solve. Tobacco products can boost mood and stimulate memory and other mental processes. Certain plants, such as mushrooms in the genus Psilocybe and marijuana, can be used as psychoactive substances. Certain foods can affect mental processes — for example, turkey has a naturally occurring chemical that makes us sleepy, and chocolate contains chemicals that lift a persons mood.

Dangers:

Substance use disorders occur when the recurrent use of psychoactive substances cause clinically and functionally significant impairment, such as health problems, disability, and failure to meet major responsibilities at work, school, or home.  Nearly all psychoactive substances have the potential for dangerous, if not fatal effects, nullifying the idea many people have when they consider some substances as "recreational" and therefore safe to ingest. According to the DSM-5, a diagnosis of substance use disorder is based on evidence of impaired control, social impairment, risky use, and pharmacological criteria.