Marijuana is illegal in most countries primarily because of a single international treaty: the 1961 Single Convention on Narcotic Drugs. This agreement, signed by nearly every nation on Earth, required member states to limit cannabis production, trade, and use “exclusively to medical and scientific purposes.” Countries that ratified the treaty were obligated to pass domestic laws restricting cannabis, and most did. The treaty’s influence is so deep that even as attitudes shift, the legal framework it created remains largely intact more than 60 years later.
The 1961 Treaty That Shaped Global Law
The Single Convention on Narcotic Drugs was adopted by the United Nations in 1961 with the goal of creating a unified international system for controlling drugs. Cannabis, along with cannabis resin, was placed in Schedule IV of the Convention, the most restrictive category. That classification recommended prohibiting even its medical use. The treaty defined “cannabis” as the flowering or fruiting tops of the plant from which resin has not been extracted, excluding seeds and loose leaves.
Specific requirements under the treaty included prohibiting the cultivation of cannabis plants for anything other than scientific and medical purposes, mandating that countries report annually on the area of cannabis cultivation, and establishing national agencies to oversee any permitted crops. Countries that signed on were expected to build these controls into their own legal systems, which is exactly what happened across Asia, Africa, Europe, and the Americas over the following decades. For many nations, the treaty provided both the template and the political justification for criminalization.
The treaty didn’t emerge in a vacuum. By 1961, cannabis had already been restricted in many places. The United States passed the Marihuana Tax Act in 1937, which imposed heavy regulations on cannabis. A common narrative holds that the Act was designed to crush the industrial hemp industry to protect synthetic fiber manufacturers, but this is a misconception: the Act did not ban hemp fiber production. The law was driven more by concerns about recreational drug use than by industrial competition. What the U.S. and other early-prohibiting nations did accomplish, however, was building enough political momentum to push cannabis control onto the international stage.
Health Risks as the Official Justification
The stated reason for placing cannabis under international control has always been public health. The World Health Organization’s Expert Committee on Drug Dependence is responsible for evaluating the risks and therapeutic potential of controlled substances, and its assessments feed directly into how cannabis is classified under the treaties. The framework is built on two considerations: a substance’s potential for abuse and dependence, and whatever therapeutic benefits it may offer.
Cannabis does carry real health risks. Regular use, particularly starting in adolescence, is associated with increased risk of dependence, cognitive impairment, and mental health complications. These risks gave international bodies a credible foundation for maintaining strict controls. At the same time, critics have long argued that the severity of cannabis’s classification was disproportionate. For nearly 60 years, the 1961 Convention grouped cannabis alongside heroin, fentanyl analogues, and other drugs considered exceptionally harmful, a comparison that increasingly clashed with the scientific evidence.
The 2020 UN Reclassification
In December 2020, the UN Commission on Narcotic Drugs voted to remove cannabis and cannabis resin from Schedule IV of the 1961 Convention, the most restrictive tier. The vote followed recommendations from the WHO’s Expert Committee, which had been under review since 2018. Cannabis is now classified alongside substances like morphine and oxycodone: recognized as carrying abuse and dependence potential, but no longer grouped with the drugs considered most dangerous.
This reclassification was significant symbolically and practically. It removed some international procedural barriers to research and development of cannabis-based medicines. But it did not legalize cannabis or promote legalization. Cannabis remains under strict international control, and the change applies only to how nations can approach medical research and pharmaceutical development within their own regulatory systems. The CND rejected further WHO recommendations that would have reclassified other cannabis-related compounds to improve their availability for medical use.
Why Countries Still Enforce Prohibition
Even as public opinion shifts in many parts of the world, the treaty framework creates powerful inertia. The International Narcotics Control Board, the independent body that monitors compliance with drug treaties, has made its position clear. In its 2022 annual report, the INCB warned that legalizing non-medical cannabis use “contravenes the 1961 Single Convention” and appears to result in higher consumption and lower perception of risk, especially among young people. The Board has reminded governments that the conventions require drug use to be limited to medical and scientific purposes.
For countries considering reform, this creates a genuine diplomatic tension. Legalizing recreational cannabis puts a country in formal violation of a treaty it signed. The INCB has noted that decriminalization and depenalization (reducing penalties without formally legalizing) offer alternative paths that stay within the conventions’ boundaries. Many countries have taken this route, softening enforcement without changing the underlying legal status of the drug.
There are also domestic political dynamics at play. In much of Asia, the Middle East, and parts of Africa, cannabis prohibition is deeply embedded in cultural and political norms. At the end of 2023, 34 countries still retained the death penalty for drug offenses. Drug-related executions were confirmed in five countries: China, Iran, Kuwait, Saudi Arabia, and Singapore. Executions for drug offenses are also believed to occur in Vietnam and North Korea, though censorship makes confirmation impossible. While these death penalties typically target trafficking rather than simple possession, they reflect a broader enforcement philosophy in which any softening on cannabis is politically unfeasible.
The Growing Split Between Medical and Recreational Use
One of the clearest trends in global cannabis policy is the widening gap between medical and recreational legalization. Dozens of countries now permit some form of medical cannabis, typically requiring a healthcare provider’s authorization and state approval for qualifying conditions like cancer, glaucoma, or multiple sclerosis. This fits within the treaty framework, since the conventions always allowed for medical and scientific use under strict controls.
Recreational legalization is a different matter entirely. Only a handful of countries, including Canada and Uruguay, have fully legalized adult recreational use at the national level. Several U.S. states have done so as well, though cannabis remains federally prohibited in the United States. Germany moved to decriminalize possession in 2024. These governments represent a small minority, and the INCB has consistently flagged their policies as treaty violations.
The practical result is a patchwork. A substance that is legally prescribed for pain in one country can carry years in prison in the country next door. This inconsistency is a direct product of the treaty system: it created a global baseline of prohibition but left enforcement and exceptions to individual nations, producing wildly different outcomes depending on local politics, culture, and priorities.
Why Reform Moves Slowly
Changing the international legal status of cannabis would require amending or replacing the 1961 Convention, a process that demands broad consensus among UN member states. That consensus does not exist. While Western democracies increasingly lean toward liberalization, many nations in Asia and the Middle East remain firmly opposed. The INCB has urged governments to study the effects of legalization further before making “long-term binding decisions,” a position that effectively favors the status quo.
The result is that marijuana remains illegal in most countries not because of a single reason but because of layered, reinforcing factors: an international treaty that requires prohibition, a UN enforcement body that actively discourages legalization, domestic political cultures that treat drug reform as risky, and genuine health concerns that give prohibition a credible (if debated) justification. Each of these factors alone might be surmountable. Together, they form a legal and political structure that changes slowly, even as the science and public opinion around cannabis continue to evolve.