When Giles, 15, came home after a party with friends from his private day school last summer, he emerged "in such bad shape I assumed he had been taking heavy pharmaceuticals. He was paranoid, having panic attacks, and had a terrible headache. The paranoia, anxiety and depression did not go for two months. It ruined the summer and the autumn term went badly".

What Giles had been smoking that weekend — when he also went without sleep and consumed large amounts of alcohol — was not the substance Sophie's generation indulgently called "wacky baccy" but skunk. This special strain of cannabis, now widely available, has a level of tetrahydrocannabinol (THC), the active ingredient, at least 10 times stronger than in natural versions of the plant.

What guidance can we offer our children about the drug? On the one hand it is being edged towards acceptability by this month's change in the law, downgrading it from a class B to a class C drug, but on the other, more and more evidence is emerging about its potential to damage youngsters' lives. Hoping that children won't come into contact with it isn't an option: Department of Health research shows that in 2002 36% of 15-year-olds had taken illegal drugs, and 42% had been offered them.

There is a "massive amount of muddle" about the forthcoming downgrading of cannabis, said Sir John Stevens, commissioner of the Metropolitan police, last week. This admission will have done nothing to reassure parents, still reeling from reports earlier in the week that some leading psychiatrists say the drug is the "number one problem" faced by the mental health services, with schizophrenia and psychosis being linked to its use, particularly by the young.

"I don't much care about the reclassification," says Professor Robin Murray of the Institute of Psychiatry at the Maudsley hospital in south London. "But they've downplayed the risk of psychosis. Adolescents who indulge in heavy cannabis use over a long period increase their risk of schizophrenia by up to seven times.

"The critical thing is the age young people start. We see youngsters who started at 12 and 13. It can be a long time before parents know about it. Look out for a fall-off in performance at school or strange friends."

"When today's parents were young, we had the Camberwell carrot," says Roger, an animal-feed salesman from Kent, referring to the outsized spliff created in the cult film Withnail and I. "It had to be that big, it was the only way you'd get a buzz." But the cannabis Roger's son Peter, 22, was using did not turn him into an amiable space cadet like Neil, the hippy in The Young Ones. In his second year at university, Peter — who had begun staying up all night obsessively writing music and smoking skunk — began exhibiting manic behaviour that ended up with him ranting outside his hall of residence one night, having trashed his room. Two years later he is on medication and suffering from depression, and Roger finds it hard to be optimistic about his son's prospects for a return to normal life.

Sophie and Roger's experience with their children concurs with the psychiatrists' findings — that it is not cannabis alone that provokes a devastating mental breakdown. Erratic sleep patterns, obsessive behaviour, traumatic family problems and worries about academic failure can all contribute and all need to be monitored.

"Vulnerable kids start on cannabis," says Darren Hall, a regional drugs adviser for the government, who has helped to set up the Talk to Frank campaign to offer information and advice to young people about drugs. "Smoking it is just one element of their behaviour.

"Parents need to take a consultative approach. Don't dictate, but do give your children information. If you don't know the facts, then visit www.talktofrank.com together [although, amazingly, Frank does not mention skunk]. There is no lower age limit. Parents can ask their primary school children if anyone has mentioned drugs to them, and say, 'Come and talk to us about it if they do'." Six per cent of 11-year-olds have already tried illegal drugs, according to the Home Office.

"And if kids compare cannabis to alcohol," advises Hall, "tell them about the harm alcohol can do. Cannabis has the same effect as if you drink too much over a long time. Don't say, no, it's not the same."

Despite this month's reclassification, cannabis remains, says the Home Office, "a harmful drug and an illegal drug". Possession is still an arrestable offence, the maximum penalty from January 29, when its class C status becomes official, reduced from five years' imprisonment to two.

"How seriously it's dealt with is at police discretion," explains a Metropolitan police spokesman about the new guidelines drawn up by the Association of Police Commissioners. Police now have the option to issue a formal warning, a simpler procedure than arrest, in order to free up time to concentrate on the class A drugs, heroin and crack cocaine.

"But the key message for children," the police spokesman adds, "is that you will probably be arrested if you are under 18 and found in possession of cannabis. We won't ignore a 14-year-old smoking cannabis — and we'd want to know where they got it from. It's possible that dealers will encourage children by saying, 'You can take it now because it's legal'. Children must be told that it's still illegal."

Harmful, illegal, dangerous, bad for your health . . . parents can tell their children all this, but the worry remains that they won't listen.

"There are subtle levers," says Hall. Such as encouraging sport. "Kids get to a certain level but if they're smoking pot they find they can't progress. Then you can say, 'So do you want to think about giving up the weed?'"

A depressing suggestion to have to make to a child before he's even taken his GCSEs. Better to use the current high level of media coverage as a basis for informed discussion. That seems to be the best hope of keeping our children free from the damage that can be inflicted by the 21st-century version of the Camberwell carrot.

Some names have been changed

The New Law

Cannabis will be reclassified from a class B to a class C drug on January 29. The maximum penalty for supplying remains 14 years' imprisonment. For possession the maximum penalty is cut from five to two years. Under new police guidance, there will be a presumption against arrest for adults, but not for youngsters. For a first offence of possession, under 18s will be arrested and given a formal warning or reprimand. Further offences will lead to a final warning or charge.