What happens in the brain?
Your brain consists of around 100 billion nerve cells that transmit signals to each other. This process releases neurotransmitters (messenger molecule molecules) . Psychoactive substances interfere with the neurotransmitter balance by either inhibiting increased release, inhibiting reuptake, or affecting both. When this balance is disrupted, your feelings and perceptions change.
Alcohol, for example, affects areas in the brain that control thought processes and movement, amphetamine and cocaine influence the stress and reward system, LSD affects, among other things, the visual center, and MDMA (Ecstasy) interferes with your emotional center.
Prolonged or excessively high doses of psychoactive substances can change your brain and therefore your psyche, causing it to become unbalanced.