Claims that cannabis is harmless have been disputed in a report that suggests the drug is becoming more powerful and could lead to long-term health damage. The study, published in the British Journal of Psychiatry, warns that cannabis can provoke anxiety and mental illness, seriously impairs driving skills, is five times more damaging to the lungs than cigarettes, weakens the immune system and may lead to rare throat cancers or fatal heart attacks. Dr Heather Ashton (Newcastle University) explains that in the 1960s a typical ''joint'' of cannabis contained 10 mg of delta-9 tetrahydrocannabinol (THC) compared to today when a joint may contain 150 mg to 300 mg. ''Cannabis affects almost every body system. It combines many of the properties of alcohol, tranquillisers, opiates and hallucinogens,'' she stresses. In a separate report in the same journal, Dr Andrew Johns (Maudsley Hospital, London) emphasised the harmful effects of the drug for those with mental problems. He warns: ''People with major mental illnesses such as schizophrenia are especially vulnerable, in that cannabis generally provokes relapses and aggravates existing symptoms.'' He also reported that 15% of users presented with psychotic symptoms or irrational feelings of persecution, including those with no history of severe mental disorders. Dr Ashton concludes that although cannabis has proven medical benefits, these must be carefully balanced against the risks.