Legalising cannabis would store up health problems for a generation and risk fuelling demand for the drug, MPs were warned by doctors.
Medical experts, giving evidence to the Home Affairs Select Committee, said too little was known about the long-term side-effects of cannabis to justify the move.
The MPs, who are reviewing drug laws, have heard several calls to legalise soft drugs. But Rob Barnett, of the British Medical Association, said cannabis had about 400 ingredients, some known to be carcinogenic. He said: "I don't think there's enough evidence that making it more readily available is safe for society."
Dr Barnett said many smokers and heavy drinkers wished they could stop. "It is the same with drug users – a lot of people who start the habit become addicted and don't have the willpower to stop."
Clare Gerada, of the Royal College of General Practitioners, said ministers should have the courage to stand up to the "something must be done" justification for drug law reform. "I see every single day, patients who use drugs and have terrible problems ... You will get more of them if you make it more legal, or less illegal."
Christine Glover, former president of the Royal Pharmaceutical Society, attacked the idea that ecstasy should be legalised. She said: "We don't want to be endorsing it or encouraging it."
The committee was also told that many family doctors, the first point of contact for most drug addicts, were ill-prepared for coping with drug-related problems.
Andrew Thompson, NHS Alliance adviser on drugs, said drug addiction was exacting an increasing toll on GPs, especially in inner-city areas, where an estimated one in 50 people used heroin or cocaine.
Dr Gerada said: "Most doctors will have no training in substance misuse ... Lack of training inevitably leads to fear and prejudices, which in turn fosters negative responses towards this patient group."Reuse content