Psychoactive substances are chemicals that affect the brain and alter mood, perception, or behavior. They can be found in various forms, including alcohol, caffeine, and illegal drugs like cocaine or heroin. These substances interact with the brain's neurotransmitters, leading to changes in how we think, feel, and act. These substances can be classified into different categories, such as stimulants, depressants, and hallucinogens. While some psychoactive substances are legal and used for medicinal purposes, others are illegal and can lead to addiction or health issues. Understanding their effects is crucial for making informed choices.