Which substances are classified as hallucinogens?

Hallucinogens are substances known for their ability to alter perception, mood, and cognitive processes, often leading users to experience visual or auditory hallucinations. The correct answer includes LSD (lysergic acid diethylamide) and psilocybin (the active compound found in certain mushrooms), both of which are well-documented hallucinogens. These substances primarily affect serotonin receptors in the brain, which significantly influences perception and experience.

In contrast, the other options listed contain substances that do not fall into the classification of hallucinogens. Cocaine and marijuana are primarily stimulants and have psychoactive effects, but they do not typically produce hallucinations. Heroin is an opioid that is mainly associated with pain relief and dependence rather than hallucinations, while alcohol and nicotine are classified as a depressant and a stimulant, respectively. Each of these substances may alter mood or perception but does not induce hallucinations like LSD and psilocybin do.