T HE DOCTOR was blunt with Hannah Deacon, the mother of an epileptic boy. He told her that she would “never” get a prescription on the National Health Service ( NHS ) for medicine based on tetrahydrocannabinol ( THC ), the psychoactive ingredient in cannabis. The British government, too, was unhelpful, maintaining in February 2018 that cannabis had no medical value—a position that it had held for over 50 years, even as the country grew and exported cannabis for medicinal use. Yet within months, it had made a U -turn, accepting that Cannabis sativa had medical uses. Eight months after Ms Deacon made her first public plea for it, her son, Alfie, got THC -based medicine on the NHS .

Legislatures across the planet have been having similar changes of heart. This may presage broader legalisation. History suggests that when medical cannabis is permitted this is often the prelude to broader recreational access.

People have exploited C. sativa for thousands of years and its medicinal use can be traced as far back at least as 400 AD . But, like other recreational drugs, it started to face restrictions during the first half of the 20th century. Fear-mongering was common. A turning-point came in the 1900s when John Warnock, a British expatriate doctor in Egypt, suggested that cannabis was responsible for a large amount of the insanity and crime in the country. When the League of Nations met in 1924 to discuss narcotics such as opium and heroin, his “evidence” of the dangers of cannabis was influential. But his methodology was dubious. Data were gathered only from patients in the Egyptian Department of Lunacy. He spoke no Arabic, and an important way to determine if patients had been users was to note their “excited” denials when asked if they had tried the drug.

Reefer madness

Then in the 1930s America was afflicted with a moral panic, as cannabis was accused of inciting violence among Mexican immigrants and of corrupting America’s children. When the international system of drug control, the Single Convention on Narcotic Drugs, was set up in 1961 at the United Nations, the use of cannabis in traditional medicine was ignored. It was treated as having limited or no therapeutic use, and as being a dangerous drug, like heroin, requiring the strictest controls.

Within the plant are chemicals called cannabinoids, similar to molecules produced by the human body, known as endocannabinoids. A wide network of receptors in the human brain and body respond to the plant and human versions of these molecules. The body’s endocannabinoid system is involved in regulating everything from pain to mood, appetite, stress, sleep and memory. So far, 144 different cannabinoids have been found in C. sativa—most of them barely understood—and new properties are being discovered all the time.

The best known are THC , the ingredient that gets you high, and cannabidiol ( CBD ), which does not and which is increasingly used as a food additive and supplement. Drug treaties have severely impeded research into cannabis. But over the years evidence from clinical trials and elsewhere has shown its efficacy in treating a range of conditions, such as muscle pain in multiple sclerosis, nausea induced by chemotherapy, treatment-resistant epilepsy and chronic pain in adults.

Helpful both in alleviating pain and in giving pleasure, pot has been wildly popular in the decades since the Single Convention and the drug-control treaties that followed it. It is the world’s most widely grown and used illicit drug. In 2017 it was produced in almost every country on Earth. The UN ’s estimates of global drug-taking put the number of users at 188m (out of a total of 271m taking illegal drugs).

Cannabis is not completely free of dangers. An overdose is unlikely, perhaps impossible, but one in ten do become addicted. And at high doses, with high-strength strains or long-term use, there is a risk of psychosis. In adolescents there is a risk of impaired brain development. But, given how much pot is smoked for fun, it is remarkable how little harm it does. And more and more countries—over 30 so far—have legalised medical cannabis (see map). In North and South America, medical use has tended to be followed by acceptance of its recreational use. Some European countries have liberalised their laws for both sorts of purpose. But Germany, France and Britain have moved to medical pot first.

Allowing medical cannabis forces governments to build regulatory structures to control the legal supply to patients. Once this happens, it seems easier for societies to accept the idea of recreational use. When grandma starts smoking pot for her arthritis, the drug has entered the mainstream.

Other arguments are also persuasive in the push for full legalisation, such as racial disparities in prosecutions, the social and judicial costs of criminalising so many users, and the profits and taxes a legal industry might generate. But that patients are suffering seems to carry more political weight than arguments from liberalisers. Perhaps nervous politicians from a generation that grew up taking drugs find wheelchairs offer convenient cover.

In America 33 states allow medical use, and 11 have legalised the recreational kind. Nationally, most of the population favours federal legalisation. By 2024 medical cannabis will be legal in all states, and recreational use will be found in almost half, predict Arcview Market Research and BDS Analytics, firms that monitor the cannabis business. Medical use is spreading weed-like across Latin America, as opposition wanes. Medical use is already found in Argentina, Colombia, Mexico, Chile, Peru, Jamaica and Uruguay.

Some governments and health insurers will cover cannabis prescriptions. Almost 16,000 German patients receive medical cannabis—mostly for chronic pain and spasticity, and some, improbably, for attention-deficit disorder. In 2017 the leading insurer approved two-thirds of requests and spent $2.7m on pot. This year the European Parliament passed a (non-binding) vote to improve access to medical pot. Even the World Health Organisation wants cannabis treated in a less restrictive way that would acknowledge its medical utility and make it easier to conduct research. Most striking of all is the arrival of medical cannabis in countries that seemed highly unlikely to relax drug laws, including South Korea, Thailand and Zimbabwe.

In the countries that accept medical use, ease of access varies. International drug treaties technically permit medical cannabis. But the body that monitors international compliance with drug treaties, the International Narcotics Control Board ( INCB ), maintains a tone of almost perpetual annoyance in its reports, arguing medical-cannabis schemes are poorly regulated and allow leakage of the drug to recreational users.

Uruguay paved the way when it legalised cannabis in 2013. But it is the reform in Canada, a G 7 member, that has done most to heighten international tension over cannabis’s legal status. Last year it fully legalised the drug. Part of its rationale was that a regulated legal trade would curb the black market and protect young people, who were buying it there. Canada’s change has caused fierce fights within the UN in Vienna, according to Martin Jelsma of the Transnational Institute, a think-tank. The country now stands accused of undermining the drug-control system. Bill Blair, a minister responsible for organised-crime reduction, acknowledges that Canada is non-compliant. “But”, he says, “it is a very principled approach.”

Attitudes towards the drug are softening around the world. But many important countries, most notably Russia and China, remain implacably opposed to reform. The lack of a global consensus prevents the rewriting of the drug treaties. Divisions are also found within the UN itself. The Human Rights Council and the Special Rapporteur on extrajudicial killings are critical of the human-rights violations that come with harsh national policies to suppress drug use, and the WHO wants a shift in the status quo. The INCB and the UN Office of Drug Control oppose change.

It may be true that licensing medical marijuana tends to lead to a broader liberalisation. But those resisting this are swimming against the current. Mexico will probably legalise this year; Luxembourg is hot on its heels and likely to become the first EU country to legalise recreational cannabis; and New Zealand is planning a referendum on the issue. It is only a matter of time before international drug treaties will come to be seen as fundamentally broken. Some worry that international law more generally will be undermined by all this rule-breaking. Mr Blair is reluctant to be drawn on how Canada might help resolve the issue.

Joint approaches

It could withdraw from the convention. But the Canadian government has already ruled this out. When Bolivia wanted to legalise the chewing of coca leaves, it withdrew from the convention and rejoined with a “reservation”. A possibility that intrigues international-policy wonks is for Canada and other law-breakers to form an “inter se” (between themselves) agreement, allowing them to modify existing drug-treaty provisions. For this to be an option, Canada will probably want to wait until the club of outlaws is bigger.

In Britain medicinal cannabis is legal but still very hard to get without an expensive private prescription. (Alfie was lucky.) The dilemma is that cannabis sits in an unusual medical no-man’s-land: neither licensed for most of the uses for which people want it, nor tested to the standards that patients usually expect from medicines. Despite this, many countries are finding ways to push forward. France, for example, is moving ahead with a large-scale clinical trial of the medical uses of cannabis.

The drug’s ambiguous legal status as a medicine will persist for years. A long history of prejudice has thwarted research and deprived millions of patients access to therapies that might help them. The work of creating regulated and approved medicines should be well advanced, but is only just beginning. Ironically, it may be that only when cannabis is legal for recreational use will a fuller picture emerge of the benefits it offers and the risks it poses. ■