Hard and soft drugs
All drugs–nicotine, cocaine, marijuana and others–affect the brain’s “reward” circuit, which is part of the limbic system. This area of the brain affects instinct and mood. Drugs target this system, which causes large amounts of dopamine—a brain chemical that helps regulate emotions and feelings of pleasure—to flood the brain. This flood of dopamine is what causes a “high.” It's one of the main causes of drug addiction.