Long-term health impact of Cannabis is far from clear

[Dragonfan]

The Independent

Jeremy Laurance, 9 February 2009

 

Just how dangerous is cannabis? Almost a year ago, the Prime Minister announced tougher new laws on cannabis possession, uprating it from a Class C to Class B drug, because of increased fears about the risks posed by Britain's most popular illegal substance.

 

His move, reversing a decision to downgrade the drug taken four years earlier, came in response to reports that the country was in the grip of an epidemic of cannabis-induced psychosis. Headlines claimed that the cannabis sold on the streets was 20 to 30 times stronger than in the 1970s and was tipping increasing numbers of vulnerable people into mental illnesses such as schizophrenia.

 

Today's report linking cannabis for the first time with testicular cancer appears to confirm the drug's dangers. But caution is needed. It is a small study and it has shown an association, not cause and effect. For once we can echo the familiar call of academics, that more research is needed.

 

But in one respect, the link with testicular cancer looks plausible; it is one of the few cancers that has risen dramatically in recent decades, in parallel with the growth in cannabis use. This is not the case for psychosis; there has been no increase in the past 30 years and rates have declined since the mid-1990s.

 

Nonetheless, concern about cannabis has focused on its impact on mental health. Doctors have been worried for a decade about the effect of the drug on a small group of vulnerable users with an inherited pre-disposition to schizophrenia. The Advisory Council on the Misuse of Drugs concluded that cannabis smoking increased the lifetime risk of schizophrenia by less than 1 per cent.

 

Earlier research published in the British Medical Journal in 2003 suggested cannabis might be responsible for up to 30,000 deaths a year, mainly from cancer and heart disease, if its ill-effects were comparable with those of tobacco.

 

It was a big "if". Although people smoked fewer cannabis cigarettes than tobacco ones, the researchers argued, the large puff volume and deeper inhalation characteristic of cannabis smoking meant each cigarette was likely to be more damaging. Tar from cannabis spliffs contained 50 per cent more carcinogens than the equivalent cigarette.

 

Critics pointed out that most cannabis users give up in their 30s, limiting their long-term exposure, which is a crucial factor in cigarette-induced lung cancer. Two long-term studies of the drug involving more than 100,000 people in total in Sweden and the US found no increase in deaths.

 

Unlike tobacco, cannabis does not contain nicotine and so is not addictive. It was also exonerated as a cause of heart disease by a study which showed no increase in calcium deposits in the coronary arteries of young adult users of the drug, a marker of thickening of the arteries that can lead to heart attacks.

 

On the basis of the existing evidence, cannabis cannot be said to be safe, but it is less dangerous than the legal drugs of alcohol and tobacco in terms of harm caused.

 

But there is an extremely important caveat. The generation that grew up in the 1960s was the first to use cannabis on a large scale and is too young to have been followed into old age. The long-term effects of cannabis are still not known.