International treaties to curb cannabis trade and use have been built on the foundations of shaky science and imperial politics, writes James Mills.

As British doctors prepare to prescribe a new generation of cannabis-based medicines and legislation before Parliament promises a relaxation of regulations on recreational use of the drug, the UK government has come under fire from the United Nations for its approach. In February, Philip Emafo, president of the International Narcotics Control Board, stated that "no government should take unilateral measures without considering the impact of its actions, and ultimately the consequences for an entire system that took governments almost a century to establish". But the long history of cannabis regulation suggests that the whole edifice of international treaties on the drug is based on questionable science and the politics of empire.

Cannabis was but a footnote to the wider negotiations over the opium trade that came to a head in the first quarter of the 20th century. League of Nations delegates meeting in Geneva in 1924 to discuss the opium trade were surprised by an impassioned speech by the head of the Egyptian group, Mohamed A. S. El Guindy. He described hashish as being "at least as harmful as opium, if not more so", and pointed out that "this substance and its derivatives work such havoc that the Egyptian government has for a long time past prohibited their introduction into the country". Without urgent action, he predicted, the drug would "become a terrible menace to the whole world".

Cannabis had been raised as an issue at the League of Nations once before, as a result of a South African inquiry in 1923. No one had seemed much interested at the time, and the US representative at the meeting, suspecting a British attempt to confuse the larger issue of the opium trade by introducing a lesser issue, had asked pointedly whether any government had actually expressed a desire for the question to be discussed. The Advisory Committee on Traffic in Opium and Other Dangerous Drugs therefore simply asked governments to provide more information on cannabis, and a questionnaire was circulated.

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It was a very different situation at the Geneva meeting. The British government was determined to keep the meeting to discussions of maximum limits on the amounts of morphine, heroin and cocaine that could be manufactured from raw opium and coca and ways to control production of the raw materials from which the drugs were made. The Americans, however, wanted to go much further and impose an agreement restricting all production of drugs to strictly scientific or medical use.

Sharply differing views on drugs, and sharply conflicting ambitions in Asia, split the British and the Americans. The Americans eventually walked out of the meeting, but not before they had given their backing to the Egyptian cause. By the end of the conference, El Guindy, profiting from US support and the ignorance of other delegations about cannabis, had succeeded in forcing cannabis onto international treaties for the first time, where it has remained ever since.

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Ironically, though, it was British reports from the Empire that provided El Guindy with the evidence on smuggling and mental health that swayed his case at Geneva. He revealed, for example, that, despite the fact that the consumption of cannabis was forbidden in his country, the Egyptian customs service had seized 3,262,2kg of hashish in the previous year alone. One of his colleagues claimed that "about 70 per cent of insane people in lunatic asylums in Egypt are haschiche eaters or smokers".

The claims reflected imperial concerns at the time. Throughout the 1920s, various British administrations had been tracking the activities of Henri de Monfried, a suspected gun-runner during the first world war. In the early 1920s, he bought 10 tons of cannabis from the authorities in Bombay, which disappeared by the time his ship arrived in Europe. He claimed to have been the victim of piracy, but the British suspected him of smuggling the drug into Egypt, a suspicion he later cheerfully confirmed in his memoirs.

This led the British colonial presence in Egypt to compile a report on the market for contraband hashish in the country. In February 1924, the European Department of the Ministry of the Interior concluded that "it is practically impossible to keep hashish out of Egypt". It found that cannabis was smuggled in boats from Greece and Syria and dropped overboard in marked rubber bags that were picked up later or landed in small boats near Alexandria and Port Said and carried inland by Bedouin on camels.

It continued: "Hashish has been found in the middle of cotton goods from Manchester, it is put in bundles of newspapers from abroad, it is hidden inside imported goods of all kinds such as tins of petroleum, bricks, millstones, marble columns, hollow bedsteads, barrels of olives, looking glasses etc" and "sometimes it is made up to resemble the inner soles of boots and the smugglers merely walk ashore with it".

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The report, which was forwarded to the Egyptian director-general of public security in Cairo, concluded that hashish traffic should be made an international issue and that the League of Nations should agree severe penalties for dealing in or consuming the drug. It was this recommendation that El Guindy advocated in Geneva.

The Egyptian claims about the effects of cannabis on mental health also owed much to British information. The country's mental health system was the personal fiefdom of Englishman John Warnock, who was appointed by the public health department in Cairo in 1895 to reform the Abbasiya Asylum and remained in post for 28 years. During this time, he expanded the existing institution, built a new hospital, drafted laws on mental illness in Egypt and created a new department dedicated to lunacy within the colonial Ministry of the Interior. By the time he retired, almost 2,500 Egyptians were being treated at any one time within the units of the lunacy department.

He seems to have developed little attachment to the place that was his home for so long. He admitted that he did not study written Arabic and that his grasp of the vernacular was such that he could only "make my wants known and give orders". He contemptuously dismissed Egyptian political ambitions after the first world war, noting that "self-determination was proving to be an infectious mental disorder". Yet despite this apparent lack of sympathy with the society around him, he felt sure that he could locate the chief cause of insanity in the Egyptian population: cannabis.

Within ten months of his arrival at the asylum, Warnock claimed that he was able to produce an authoritative account of the causes of mental illness there. He cited statistics showing that in 41 per cent of all his male patients, hashish alone or combined with alcohol caused mental illness and concluded: "I have no doubt that in quite a number of cases hashish is the chief if not the only cause of the mental disease." In 1924, just after his retirement, he wrote that "with the men the attack of insanity was attributed in nearly all cases to one of three causes, the use of hashish, some disappointment or grief, and religious excitement. Of these, the first is by far the most frequent." But despite his lack of knowledge about Egypt and Egyptian society, and although he conceded that "only a comparatively few" smokers of hashish suffered "grave toxic symptoms", it was his statistics that were the sole "scientific" prop for El Guindy's attack on cannabis at Geneva.

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The history of cannabis' inclusion in international drugs treaties may provide an amusing perspective on the tangled webs spun by the British Empire in its decline, but it is not clear that it offers much evidence that the international system of cannabis control was well founded or clearly thought-out. It is equally unclear whether it provides sufficient reason for a 21st-century home secretary to take heed of a UN body that has its origins in such a tale of shaky science and long-forgotten imperial politics.

James Mills is an Economic and Social Research Council fellow and senior lecturer in the department of history at the University of Strathclyde.

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Cannabis Britannica: Empire, Trade and Prohibition is published this week by Oxford University Press, ?25.00.