A psychoactive substance is any chemical that affects the brain's function, leading to changes in mood, perception, or behavior. These substances can be found in various forms, including legal drugs like caffeine and alcohol, as well as illegal drugs such as cocaine and heroin. They work by altering the way neurotransmitters communicate in the brain. Psychoactive substances can be classified into different categories, including stimulants, depressants, and hallucinogens. Stimulants, like amphetamines, increase alertness and energy, while depressants, such as benzodiazepines, promote relaxation and reduce anxiety. Hallucinogens, like LSD, can cause altered perceptions of reality.