Extracts of the hemp plant, Cannabis sativa, which grows freely in temperate and tropical regions, contain the active substance delta-9-tetrahydrocannabinol (THC). Marijuana is the name given to the dried leaves and flower heads, prepared as a smoking mixture; hashish is the extracted resin.

For centuries, these substances have been used for various medicinal purposes and as intoxicant preparations. Marijuana was brought to North America by immigrants, mainly in the 19th century, and began to be regarded as a social problem in the early years of this century; it was banned during the 1930s. Its use increased dramatically in the 1960s, and recent figures suggest that about 15% of the adult population in America and Western Europe have taken cannabis at some time, with a much higher proportion (close to 50%) among teenagers and young adults.

THC acts mainly on the central nervous system, producing a mixture of psychotomimetic and depressant effects, together with various centrally mediated peripheral autonomic effects. The main subjective effects in humans consist of:
- A feeling of relaxation and well-being, similar to the effect of ethanol, but without the accompanying aggression.
- A feeling of sharpened sensory awareness, with sounds and sights seeming more intense and fantastic.

These effects are similar to, but usually less pronounced than, those produced by psychotomimetic drugs such as LSD. Subjects report that time passes extremely slowly. The alarming sensations and paranoid delusions that often occur with LSD are seldom experienced after marijuana.

Central effects that can be directly measured in humans and animal studies include:
- Impairment of short-term memory, and simple learning tasks - subjective feelings of confidence and heightened creativity are not reflected in actual performance.
- Impairment of motor coordination (eg. driving performance).
- Catalepsy - the retention of fixed unnatural postures.
- Analgesia.
- Anti-emetic action.
- Increased appetite.

The main peripheral effects of marijuana are:
- Tachycardia, which can be prevented by drugs that block sympathetic transmission.
- Vasodilatation, which is particularly marked on the scleral and conjunctival vessels, producing a bloodshot appearance characteristic of marijuana smokers.
- Reduction in intra-ocular pressure.
- Bronchodilatation.

Tolerance to cannabis, and physical dependence, occur only to a minor degree, and mainly on heavy users. The abstinence

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