Stimulants: drugs which elevate mood, increase wakefulness and give an increased sense on mental and physical energy.e.g caffeine.
Opiates: It is often called narcotic analgesics because they are strong painkillers and because they produce a feeling of euphoria and sleepiness.
Hallucinogens: Hallucinogens are among the oldest known drugs that have been used for their ability to alter human perception and mood.
Most drugs of abuse can be placed in the above categories although some marijuana, cannabis, nicotine and solvents must be considered separately as they do not fit into the above categories.
Marijuana
As marijuana enters the brain, it causes the user to feel euphoric—or high—by acting on the brain's reward system, which is made up of regions that govern the response to pleasurable things like sex and chocolate, as well as to most drugs of abuse. Marijuana activates the reward system in the same way that nearly all drugs of abuse do: by stimulating brain cells to release the chemical dopamine.
Along with euphoria, relaxation is another frequently reported effect in human studies. Other effects, which vary dramatically among different users, include heightened sensory perception (e.g., brighter colors), laughter, altered perception of time, and increased appetite.
After a while, the euphoria wears -off and the user may feel sleepy or depressed. Occasionally, marijuana use may produce anxiety, fear, distrust, or panic.
Marijuana users who have taken large doses of the drug may experience an acute psychosis, which includes hallucinations, delusions, and a loss of the sense of personal identity. Short-term psychotic reactions to