I started smoking it in my early twenties. I gave up in my fifties for
the sake of my lungs.

In my fifties I suffered a variety of medical problems which seemed to
be degrading my mental powers. My IQ was tested by a psychologist and
found to be within a couple of points of my IQ as measured at age 12
and age 20. Since IQ's are adjusted to compensate for age-related
decline that means I was performing at the level expected for my age,
although down a bit from youthful IQ.

I later discovered it wasn't my medical problems which were degrading
my mental powers, it was the drugs I was being prescribed for
them. When I stopped the drugs I got a very useful recovery in mental
powers, in memory, concentration, perspective, acuity, etc.. What
doctors and psychologists had been insisting was a natural age-related
decline turned out to be due to the drugs they were prescribing.

So I'm pretty sure that when undrugged (by prescription meds) my IQ in
my fifties was better than expected, i.e., had declined less than
expected for my age. It certainly didn't stop me getting a postgrad
degree and winning academic prizes in my mid forties.

While under the influence, of course, but my decades of smoking the
stuff co-incided with considerable development of both my writing
abilties and my public speaking abilities.

I know people like that who smoke pot. I know people like that who
spend their evenings boozing. I know people who don't require the
assistance of any drug to degenerate. I also know professors,
well-known journalists, polticians, and successful novelists, who use
pot.

Of course it may well be bad for some people. Alcohol as it happens is
pretty bad for me. It has much more deleterious effects when I drink
it, both phyical and mental, and the bad effects last much longer,
than with pot. I also have a tendency to become addicted to it, which
may be a genetic thing since I'm descended from alcoholics on both
sides of the family.

I realise I'm untypical, but if legislation were to be based on my
personal experiences I'd say alcohol should be banned and pot made
legal. I've also known some very smart professors who could run
intellectual rings round most of their colleagues despite having spent
their lives drinking enough to be suspected of having a serious
alcohol problem.

It looks to me as though whether this or that drug degrades your brain
is very much a YMMV thing. It wouldn't surprise me if we find out that
there are infections and common prescription drugs which slightly
knacker the brain semi-permanently, and if such things as eating fish
and taking both mental and physical exercise considerably improve the
brain's recuperative powers.

Don't confuse correlation with cause and effect.  That's the typical
fallacy that prohibitionists use (and which led to the "crack baby"
myth, for example).

For a thorough unbiased review of marijuana by two independent
scientists, see: _Marijuana Myths, Marijuana Facts_, by John Morgan, MD,
Lynn Zimmer, PhD. New York: Lindesmith Center, 1997.

As Morgan and Zimmer put it:

"The National Institute on Drug Abuse funds research to find harm from
marijuana...then disseminates the findings.  Very modest findings are
presented as "significant."  Statistical associations...are used to
imply a causal relationship.  Studies showing no effect--or a positive
effect related to marijuana--are ignored completely."
--John Morgan, MD, Lynn Zimmer, PhD. _Marijuana Myths, Marijuana Facts_,
New York: Lindesmith Center, 1997.