Gateway drugs are milder, accessible substances that can lead to more severe drug use. They include alcohol and cigarettes. Depressants are downers that slow brain and nervous system functioning, giving users feelings of pleasure and relaxation but later sadness and indifference. Their effects are similar to stimulants but opposite. Stimulants are uppers that temporarily make people alert but later physically tired and sad. Hallucinogens change perceptions of time, color, and sound and can disrupt thinking and communication, leading to withdrawal symptoms when no longer used.