Crazy and, above all, hazy history of a most imperial weed

At a time when the legalisation of cannabis is hotly debated, a book that critically explores the history of cannabis is welcome. James Mills opens with the British encounter with cannabis in the early 19th century, focusing on the first systematic investigation of the drug by W. B.

O'Shaughnessy in late-1830s Calcutta, whose praise for cannabis as a pain-killer and "anti-convulsive" alerted the West to its medical possibilities. If, as Mills suspects, O'Shaughnessy used cannabis as a rather sensational way of furthering his career, he was certainly not the last person to do so.

The colonial government, however, was less interested in the medicinal value of cannabis than in the ways in which the market for ganja could be taxed. Mills quotes at length from contemporary reports on the cultivation of hemp and the manner in which forms of the drug were prepared. But his evidence for a "booming cannabis economy" (and hence the colonial incentive to protect the trade from smugglers and abolitionists) is sketchy. More strikingly, Mills shows how, on the basis of scant evidence and a great deal of cultural stereotyping, British doctors in India identified cannabis with insanity, claiming that a large proportion of asylum inmates were cannabis users and that their mental state was directly related to its use.

From mid-century, cannabis enjoyed a certain vogue in British medicine, being used to relieve labour pains, combat tetanus and even, ironically, to treat insanity. But opposition to cannabis was growing. Not only were doubts expressed about its efficacy and hallucinogenic side-effects; it was also caught up in the crossfire of the late 19th-century anti-opium debates, one MP claiming that "deadly bhang" was "the most horrible intoxicant the world has yet produced". In 1893 the British government agreed to an official inquiry, but the Indian Hemp Drugs Commission denied any clear connection between moderate or even habitual use of cannabis and mental illness, quelling criticism for more than two decades.

The real turning point came in the interwar years. In 1924, at a League of Nations conference on opium, the Egyptian delegate startled the British by calling for the suppression of the international trade in cannabis as a "scourge which reduces man to the level of the brute and deprives him of health and reason". Anxious to placate opinion, the British began for the first time to contemplate measures to restrict the hemp drugs trade.

Opinion was turning against cannabis in Britain, too. In 1923 two African waiters were arrested for attempting to supply what was at first alleged to be opium, but turned out to be "hashish", a substance then barely known to the police. The trial that followed sensationalised the threat posed by cannabis and, although the Home Office remained sceptical, the Pharmaceutical Society was panicked into adding the drug to its Poisons Schedule, thus making its sale subject for the first time to legal control.

Mills convincingly argues that much of what has been seen as hard evidence about cannabis is in fact far from reliable, and that the move to make cannabis illegal arose not in response to widespread use of the drug (which did not occur in Britain before the 1950s) or to a careful review of the facts, but from pressure resulting from an internal opium conference and a bungled police case. Cannabis policy has been "driven by mistakes and misunderstandings rather than by well-informed debate".

The intrinsic interest and public importance of the cannabis story makes this a significant contribution to the debate. There have been histories of cannabis before, but never has the record been as thoroughly examined. But if on those grounds the book deserves to be widely read, on others it appears less satisfactory. Why end in 1928? A book of barely 200 pages could easily have been expanded to carry the history down to the present day rather than being carried over to a second volume. The pace and direction of Cannabis Britannica is erratic: at times we wallow in pages of undigested testimony, while at others we shoot through important issues without a pause. Although India (and more fleetingly Egypt) figure centrally in this story, there is little reflection on the production and uses of "colonial knowledge" or why ideas of race intersect with images of madness. At times, too, it is unclear whether this is to be read as a history of what was said and done in certain circumstances or whether we are being presented with evidence that still, in the 21st century, deserves our careful consideration.

David Arnold is professor of South Asian history, School of Oriental and African Studies, London.