The Big Question: So how dangerous is cannabis?

Why are we asking this question now?

The head of the UN's anti-drugs office has said that cannabis use has turned into a major pandemic which is causing as much harm as cocaine and heroin. Antonio Maria Costa also implicitly criticised countries such as Britain for relaxing the law on the possession of cannabis.

"Policy reversals leave young people confused as to just how dangerous cannabis is," Mr Costa said. "With cannabis-related health damage increasing, it is fundamentally wrong for countries to make cannabis control dependent on which party is in government. Today, the harmful characteristics of cannabis are no longer that different from those of other plant-based drugs such as cocaine and heroin."

What is cannabis?

The most commonly used illicit drug in Britain, if not the world, also called marijuana, it is produced from certain parts of the Cannabis sativa plant and comes in various forms - dried leaves, concentrated resin known as hashish, or distilled oil. The strongest parts of the plant are the female flowering tops, which are prevented from going to seed by growing them in a pollen-free environment. Sensemillia, as this form of cannabis is sometimes called, is strong because none of the plant's energy goes into making seeds, but instead produces the psychoactive substances which cause the desired effect.

Cannabis grows wild in many parts of the world, from Poland and Hungary to Afghanistan, India and China. Its dried leaves or resin have been smoked by varied cultures over many thousands of years. Cannabis has been used in societies ranging from the Hindus of India, the Thracians of southern Europe and the ancient Scythians, who liked to smoke it in a steam room. Indeed the charred seeds of cannabis have been found at a Stone Age burial site in Romania, and cannabis was first documented as a herbal remedy in a Chinese pharmacy text of the first century AD.

Why do people take it?

Cannabis is a psychoactive substance; in other words, it affects the brain. But it also affects other parts of the body. It increases pulse rate, decreases blood pressure, causes bloodshot eyes and increases appetite. However, it is the effects on the brain that cause the feelings of calm euphoria and gentle elation that many users enjoy.

The drug has a mild sedative effect but the experience depends greatly on individual mood and the social environment at the time it is taken. Some people get the giggles and become talkative, others become subdued and quiet - the classic symptoms of being "stoned". Many people feel less inhibited while under the influence of cannabis, in much the same way as drinking alcohol, which is why it is a common party drug.

What does it do to the brain?

The most active ingredient of cannabis is a chemical called tetrahydrocannabinol (THC). It can pass across the biological barrier that separates the brain from the bloodstream, and in doing so penetrates the central nervous system. Here, it acts on the natural proteins or receptors that control nerve impulses passed from one part of the brain to another.

Cannabinoids such as THC act on a specific protein receptor that is widely distributed in the brain. As a result it interferes with concentration and thought, memory, pain perception and muscle co-ordination. THC particularly interferes with the cerebellum - the "autopilot" of the brain - which is important for balance, posture, and co-ordination of movement. The drug also affects the hippocampus, which is important for the formation of memory.

These influences on the brain help to explain why cannabis intensifies ordinary sensory experiences, such as eating, watching films or listening to music. They also explain why users get a false sense of how time passes, and why they suffer from various problems with short-term memory, poor reaction time and general unsteadiness.

Are there any more dangerous side-effects?

This is a hotly disputed topic. Cannabis does not produce physical dependency, as does heroin, but some people who use it regularly can become psychologically dependent. Cannabis smoke is carcinogenic, and so can contribute to lung cancer, just like tobacco smoke. And regular smoking can exacerbate existing respiratory problems, such as asthma, bronchitis and wheezing.

A few studies have suggested that regular users may also have impaired immune systems, and there is little doubt that driving while stoned is dangerous - one study found that smoking cannabis doubles the risk of fatal car crashes. According to a study in The Lancet, large doses of THC produce confusion, amnesia, delusions, hallucinations, anxiety and agitation. "Such reactions are rare, occurring after unusually heavy cannabis use; in most cases they remit rapidly after abstinence from cannabis," it says.

The really important issue is whether cannabis can cause serious, long-term problems for a person's mental health. Earlier this year the Advisory Council on the Misuse of Drugs concluded that there may be a link between cannabis use and the onset of psychotic illnesses, although there was insufficient evidence to state that frequent users were more likely to develop schizophrenia. Nevertheless, several studies have suggested that there may well be an association between smoking cannabis in adolescence and mental illness in later life - including schizophrenia.

What does the stronger version do to you?

It has been suggested that the smoking of more potent forms of cannabis, known as "skunk", can result in something called cannabis psychosis, although this has not been conclusively proved. Skunk is a generic name used to describe the 100 or so varieties of cannabis plant that have higher-than-average levels of THC. Skunk may also contain higher levels of the 40 other substances in cannabis that are thought to be capable of having an effect on the body.

Traditional varieties of cannabis have a THC content of between 2 and 4 per cent, while some varieties of skunk can have THC levels of up to 20 per cent. Some users say the immediate effect of smoking skunk is that they get stoned more quickly. They also report higher levels of transient hallucinations, which are particularly common in people who have already taken LSD, a known hallucinogenic drug.

Skunk's potency can sometimes catch people out if they have been used to more dilute forms of cannabis. As a result they can suffer from anxiety attacks and feelings of mild paranoia. However, there is no evidence to suggest that smoking skunk poses any new risks compared with the heavy smoking of weaker forms of cannabis.

Is cannabis medicinal?

Cannabis has a long history as a folk remedy, and some of its natural constituents are reported to have therapeutic value for illnesses such as asthma, glaucoma, mild to severe muscle spasms and pain, as well as anorexia and mood disorders.

Is smoking cannabis really that bad for you?

Yes...

* There is convincing evidence to suggest a link between heavy use and serious mental illness

* Cannabis smoke is just as dangerous as cigarette smoke in causing lung disease

* Driving while even mildly stoned significantly increases the risk of fatal accidents

No...

* There is no evidence that cannabis causes physical dependency in the manner of heroin or cocaine

* Mild users of cannabis are not more likely to become addicted to 'harder drugs'

* Many long-term users of cannabis lead normal, healthy lives which they find enhanced by recreational use of the drug