A BIT DOPEY?

I have not read the report on which the article on cannabis and brain
damage was based (9 March, p 6), so some of these comments may be off the
mark. But if one group had smoked cannabis for an average of 24 years,
another for 10, and a third had never smoked, it's likely the more
experienced smokers were older.

Did the testers control for age? I'm into my sixties, and I can vouch for
the fact that an addtional 14 years of breathing slows down one's cognitive
processes too.

To be valid, the test doses must reflect the usual amount a smoker will
use. Two joints a day does not represent common use. Was the use of other
drugs (both legal and illegal) controlled for? If the subjects had used
other illegal drugs, is there any assurance they actually were the
purported drugs? Was the heavy use an attempt to self-medicate? If so, then
the problems may have been the root cause, not the result, of use.

If one's sample is drawn from people who show up for mental health
treatment, it isn't surprising to find they may be depressed, or have
cognitive problems. They should be compared with 10-year smokers and
non-smokers who also show up for mental health treatment.

Was healthcare constant? Lower socio-economic status is likely to mean
poorer healthcare. People with higher incomes are likely to have better
healthcare and seek private treatment, and so not show up in a sample.

Thomas Roberts, Northern Illinois University, DeKalb, Illinois