When Harvard University psychiatrist Dr Lester Grinspoon began researching marijuana in 1967, he was in no doubt it was a very harmful drug. With an aim to define scientifically the nature and dangers of the drug, he reviewed all the scientific, medical and lay literature available. By the time he'd finished his research two years later, he was convinced that not only was cannabis far less harmful than he had originally believed, but that no harm it might cause was nearly as serious as the damage attributed to the annual arrest of hundreds of thousands of mostly young people. A major contribution to the debate on legalising cannabis.