DON'T EXAGGERATE THE DANGERS OF CANNABIS

Not that cannabis is a safe drug, because there is no such thing, but
it is not "more dangerous than heroin" (Melanie Reid, October 19). We
assert that even though one of us has published work showing heroin
not to be as dangerous as commonly thought. It is not even true that
cannabis is more dangerous than alcohol.

The evidence, misrepresented in the article, is that starting to use
cannabis under 15 makes subsequent psychotic symptoms and
schizophrenia more likely among people susceptible to such problems.
There may be a genetic component. Robin Murray has made a substantial
and sophisticated contribution to the relevant research that deserves
better coverage than given here. One recent paper that Murray
co-authored concluded: "Although the regular use of cannabis may
increase risk of psychotic symptoms, most of those who use cannabis
regularly do not develop psychosis and most cases of psychosis are
not attributable to cannabis."

Schizophrenia is, thankfully, rare, while cannabis use is very
common, so the increase in risk, while statistically significant, is
very small in the general population. In one key study, cannabis use
before age 15 was associated with three additional cases of
schizophrenia in 759 people. Links between alcohol and mental and
physical health are much stronger.

A different issue is that cannabis also poses a problem for people
with mental health problems, as it can interact badly with medicine,
worsen symptoms or contribute to them ceasing to take medication.
Many people with mental health problems do take cannabis, but to feel
better, reduce boredom and because their peers do; there is no cause
and effect. To date there is no evidence that somebody who begins to
use cannabis older than 15 and who is not predisposed to mental
illness will suffer any mental health effects. It is important not to
muddle psychotic symptoms (such as seeing things that are not there)
and schizophrenia. Psychotic symptoms can occur without mental
illness or current drug use, in bereavement for example.

Melanie Reid's article makes it seem that any young person who uses
cannabis is at risk of becoming mentally ill. This is untrue. Some
40% of people under 35 have taken it at some point, as well as older
people, including some politicians (who usually did not enjoy it or
inhale), and many other respectable and largely untroubled people.
The only accurate part of the article is that there is not enough
research on cannabis, given that it is so widespread and popular.

There is a need to reduce drinking, smoking and drug use among the
under-16s. This is not helped by scare stories. Remember that
cannabis is considered beneficial for a range of medical conditions
and is licensed for medical use in 11 US states.

Richard Hammersley, Professor of Health Psychology; David Shewan,
Department of Psychology, Glasgow Caledonian University.