Melanie Philips is a professional scribbling right-wing wind-up merchant, she is rarely, if ever, qualified in the areas and topics she spouts off about, at length.

Melanie Phillips, professional angry ar$ehole.

The other main concern about the chronic use of cannabis is whether it can
lead to mental illness (especially schizophrenia). Although debated for well
over a century, no clear causal link has been demonstrated. The onset of
schizophrenia often occurs in the late teens, when cannabis use is most
common, so that an association is inevitable. This does not, though,
necessarily mean that the relationship to cannabis is causal. To make the
interpretation of such findings more difficult, many of these individuals
have used other drugs such as amphetamines that may also precipitate
schizophrenia. Moreover, as discussed in paragraph 4.3.7, cannabis
intoxication can itself lead to psychotic symptoms that may be mistaken
for schizophrenia.
4.4.7 On the other hand cannabis use can unquestionably worsen schizophrenia
(and other mental illnesses) and lead to relapse in some patients. Its use
should therefore be particularly discouraged in all people with mental health
problems. We do not know why those with schizophrenia use cannabis when
it can make their condition worse. It may be cultural or related to peer
pressure; but it is also possible that cannabis helps deal with some aspects
of the illness, or possibly ameliorates some of the adverse consequences
of their medication.

So there are a number of reason a causal relationship is hard to establish.

http://drugs.homeoffice.gov.uk/publication-search/acmd/cannabis-class-misuse-drugs-act?view=Binary

On comparing alcohol with cannabis, I think Mark is right. The fact is that alcohol is legal, and there is no legal limit to consumption of alcohol (so long as no one is driving). So one pint may well be less harmful than one ecstasy tablet, but the point is that people can freely drink 8 pints if they really want to.

It would be like saying that cars aren’t dangerous to pedestrians because they can all travel under 30 mph if they want, when drivers are free to drive up to 70 mph depending on the road.