http://www.guardian.co.uk/science/2004/jun/17/thisweekssciencequestions
There is evidence to suggest that is is psychologically, if not physiologically.
According to the Observer, drug treatment centres are reporting a rise in the number of cannabis cases they are dealing with. Nine per cent of all those attending clinics cited cannabis as the main reason for seeking treatment, twice as many as a decade ago.
Michael Rowlands, medical director at the Priory Farm Place, says cannabis shows all the classic signs of dependency.
“There’s a strong desire to use, which overrides other activities, so friends and hobbies and work are neglected,” he says. “There’s difficulty in controlling the amounts you use. There’s a degree of tolerance developed so you need higher doses to have the same effect. And then you persist in using despite the fact it’s causing you ill health or debt.”
The main thing that separates cannabis from heroin or nicotine is that the physical withdrawal state is not normally as severe.
Almost all addictive drugs stimulate a part of the brain – called the mesolymbic dopamine system – that acts as a reward pathway in the central nervous system. Receptors for the active ingredients in cannabis have been found in this system. Once stimulated, these receptors begin a cycle of reward that can lead people to use more of the drug.
Rowlands says the apparent increase in cases of addiction might be nothing more than a product of the changing attitudes towards cannabis use. “Some of the stigma is going. People are much easier at talking about addiction,” he says. “There are vast numbers of people taking cannabis. Some of them, 8 to10%, will get some type of dependency.”
More concerning than any apparent rise in addiction is the potential to cause psychoses in heavy users.
Robin Murray, a psychiatrist at King’s College London, is one of Britain’s leading researchers in this area and his results are worrying. “The conclusion was that, if you took cannabis at age 18, you were about 60% more likely to go psychotic. But if you started by the time you were 15, then the risk was much greater, around 450%,” he says.