The message
from Jaques d'Alltrades contains these words:

It has emerged that teenage smokers of cannabis can contract a type of
dementia. (IIRC, You And Yours, BBC R4 - was this week, I think, so
ought to be extant on their wibble)

Not just teenagers; and it's not dementia, it's cannabis-induced
psychosis.The majority of users won't develop it, but there is no way of
predicting who is at risk. Unlike alcoholism it isn't linked to
prolonged or excessive use, there are no warning signals, and you don't
sober up as the drug level in the blood reduces.

The broad difference between dementia and psychosis is that dementia is
usually gradual, progressive and permanent brain deterioration;
sufferers' families usually spot it developing and have chance to
protect the person before they are in physical danger. A psychotic
episode can develop very fast out of the blue and inspire
life-threatening behaviour.

If the person is very fortunate,it may be a one-off episode from which
they make a full recovery eventually, though it may take a considerable
time.Not all people are so lucky. There is a theory that recurrence of
cannabis-induced psychoses may trigger schizophrenia in some susceptible
people.

Psychotic episodes involving dangerous behaviour often result in the
person being detained in mental hospital without their consent; which
can itself be a very difficult experience for young people.

Janet.