Cannabis has psychoactive and physiological effects when consumed. Aside from a subjective change in perception and, most notably, mood, the most common short-term physical and neurological effects include increased heart rate, increased appetite and consumption of food, lowered blood pressure, impairment of short-term and working memory, psychomotor coordination, and concentration. Long-term effects are less clear.

While many psychoactive drugs clearly fall into the category of stimulant, depressant or hallucinogen, cannabis exhibits a mix of all properties, perhaps leaning the most towards hallucinogenic or psychedelic properties, though with other effects quite pronounced as well.

There are no verified human deaths associated with cannabis overdose.

Marijuana is the most commonly used illicit drug (15.2 million past-month users) according to the 2008 National Survey on Drug Use and Health (NSDUH). That year, marijuana was used by 75.6 percent of current illicit drug users (defined as having used the drug sometime in the 30 days before the survey) and was the only drug used by 53.3 percent of them.

Cannabis can be habit-forming and the development of cannabis dependence in some users has been well established; its effects on intelligence, memory, respiratory functions and the possible relationship of cannabis use to mental disorders such as schizophrenia, psychosis, Depersonalization disorder and depression are still under discussion.

The high lipid-solubility of cannabinoids results in their persisting in the body for long periods of time. Even after a single administration of THC, detectable levels of THC can be found in the body for weeks or longer (depending on the amount administered and the sensitivity of the assessment method). A number of researchers have suggested that this is an important factor in marijuana’s effects, perhaps because cannabinoids may accumulate in the body, particularly in the lipid membranes of neurons.

Cannabis is consumed in many different ways, most of which involve inhaling vaporized cannabinoids (“smoke”) from small pipes, bongs (portable version of hookah with water chamber), paper-wrapped joints or tobacco-leaf-wrapped blunts. Alternatively, the cannabis plant flowers may be finely sifted producing kief, a powder especially rich in the oil-glands or trichomes which contain the highest amounts of cannabinoids.