I am very sorry to hear about your son. Obviously we can’t discuss his own case in detail, as I haven’t met him, but we can draw general conclusions from the points you raise. These may not all apply directly to your son. I would always be very careful about treating a patient with possibly psychotic symptoms, and a heavy cannabis smoker and therefore in danger of having a psychotic breakdown, with Prozac. In these cases I feel that the treatment should have at least an element of antipsychotic therapy. SSRIs, of which Prozac is one, can precipitate psychotic symptoms in vulnerable patients. In the recent Panorama programme on the effect of SSRIs on some patients who were depressed, the conclusion of the experts who analysed the cases was that when there had been disaster it wasn’t so much the drug, but its misprescription, that was at fault.

I took part in the discussion after the programme and it was worrying that so many patients in whom the depression was part of the negative symptomatology of a psychosis were treated with an SSRI without any thought of providing an antipsychotic. Little wonder that occasionally people had become aggressive or suicidal.

I can’t advise you on literature. However one of the best pamphlets on cannabis was produced by the National Institute of Public Health in Sweden - Adverse Health Consequences of Cannabis Use, A survey of scientific studies into the range of damage to health caused by cannabis. It was produced by Socialstyrelsen, The National Board of Health and Welfare in Sweden. It has been translated into English.

My boyfriend smokes cannabis every night to help him "relax". I always tell him how bad it is for him; it has caused many arguments and almost broken us up many times. He tells me I should stop trying to control him but he obviously has a psychological (if not physical) addiction both to the cannabis and the tobacco he mixes it with, and he’s been smoking more and more since he moved in with his current flatmates. He is doing a fantastic PhD but he is going into work less and less because he feels lethargic in the mornings. He is blaming the PhD, saying it is boring. How do I make him see it's the weed causing the problem and not the PhD? Kate Palmer, London

Nobody doubts that cannabis gives rise to psychological dependence. There is discussion about the degree of physical dependence, but no argument that heavy use of it leads to tolerance. There are withdrawal symptoms when cannabis is discontinued. These include irritability, sweating, headache and abdominal upsets. I would suggest that the first move is to encourage your son to leave his present flat. It is hard to renounce any habit if all those around you are carrying on with it.

The more I talk to schoolteachers and university lecturers, the more often I hear the story of the bright student whose performance falls away as the result of heavy cannabis smoking. A problem is that a cannabis smoking student rarely has insight, and either ignores his deteriorating performance or blames other factors. Girlfriends are about the only close friends and relatives who might have the power to persuade him to change.

I am a migraine sufferer and for ten years I took paracetemol plus antihistamine before bed, which prevented me from having disturbed sleep and waking with a migraine in the morning. I don't know the danger of doing this for ten years; my GP says it’s OK but I have reservations. I started to use cannabis instead, eating 2g of buds before bed. It worked a treat and after nine months my supplier went away and I was about to go back on the paracetemol but the symptoms had gone. Of course this may have been spontaneous and I make no claims about the efficacy of cannabis. It is the relative risk between cannabis and paracetemol that I am interested in. Alastair McGowan, Cardiff

Some migraines will improve with a tranquilliser and analgesic, whether paracetamol plus anti-histamine or cannabis. I would agree with your GP that the anti-histamine and small dose of paracetamol wouldn’t do any long-term harm, although sedative anti-histamines tend to give rise to a slight hangover the next day, so that you may feel depressed.

The risks of a daily dose of cannabis are considerably greater. Cannabis risks can be divided into two, the physical and mental. The worst mental risk is that it will cause breakdown in someone who has a genetic vulnerability to schizophrenia or bipolar disorder (manic depression). This recent research has shown that this genetic liability affects one in four of the population, but the chance of this susceptibility being unmasked varies according to the dose, the genetic make up of the person and other factors. Regular cannabis taking even without producing a psychotic reaction may induce a cannabis smoker’s personality – being excessively laid back, casual, forgetful, unambitious to an unusual degree and, in fact, generally idle but self-satisfied!

Cannabis smokers are much more liable than other smokers to develop cancer of the lung, head and neck (including the mouth), and less frequently some other malignancies. There have been several reports for adverse effects on the cardiovascular system, long-term ill effects on vision, damage to sperm and to pregnancies.

In addition to the factors you mentioned, psychosis caused by cannabis use carries the stigma of mental illness and is something that individuals and families may want to put behind them rather than shout about. Also, they are too knackered dealing with the problem to campaign. Our student son had a manic episode last summer, quite possibly triggered by cannabis. What can we do to help publicise this? Are there any campaigns we can add a voice to? Name and address withheld

A very good point. Parents may well want to keep quiet about the whole business. I should write to your MP. It is very interesting to study the background of the people appointed by this government to the advisory council for the misuse of drugs. It is quite extraordinary how many of them have been openly in support of the liberalisation of the laws relating to cannabis. It is equally extraordinary that none of the great experts on schizophrenia have been included in the committee even though it and crack are the two most common causes of acute psychotic breakdown. As well as writing to your MP, it would be as well to write to your local paper.