101 Years of Cannabis Prohibition: How a 1925 Treaty Changed Everything

In 2025, the world quietly passed a milestone that most people didn't notice: the centenary of cannabis prohibition. One hundred years since the International Opium Convention of 1925 first placed "Indian hemp" under international drug control—marking the moment cannabis acquired, as historians put it, "a marked world character" in the eyes of global law.

Now, in 2026, we're in year 101 of a policy experiment that has shaped criminal justice systems, destroyed millions of lives, created underground economies worth hundreds of billions, and is only now—slowly, unevenly, incompletely—being reversed. Understanding how we got here is essential to understanding where we're going.

Before Prohibition: Cannabis as Medicine and Commerce

For most of human history, cannabis was unremarkable. It was a fiber crop (hemp), a medicine, and yes, an intoxicant—but it existed alongside dozens of other plant-based substances that societies used without particular moral panic.

In 19th-century America, cannabis was freely available at drug stores in liquid form. Pharmaceutical companies including Eli Lilly and Parke-Davis sold cannabis tinctures for pain, nausea, and anxiety. It was listed in the United States Pharmacopoeia from 1850 to 1942. Doctors prescribed it. Patients used it. Nobody went to prison for it.

The same was true globally. Cannabis had been used medicinally and recreationally across Asia, Africa, and the Middle East for thousands of years. Hemp was one of humanity's oldest cultivated crops. There was nothing inherently controversial about the plant.

The Turn: 1910-1937

The shift toward prohibition happened gradually, driven by a combination of racial anxiety, bureaucratic ambition, and international politics.

In the United States, cannabis prohibition was intimately tied to anti-Mexican sentiment. As Mexican immigrants arrived in border states during the Mexican Revolution (1910-1920), they brought recreational marijuana use with them. American media and politicians seized on "marihuana"—deliberately using the Spanish name to associate it with a feared foreign population—as a tool of racial panic.

Between 1914 and 1925, twenty-six states passed laws prohibiting cannabis. These laws were overwhelmingly enforced against Mexican-Americans, Black Americans, and other marginalized communities—a pattern that would persist for the next century.

1925: The International Moment

The 1925 International Opium Convention in Geneva added "Indian hemp" to the list of internationally controlled substances. This wasn't originally on the agenda—Egypt and South Africa pushed for its inclusion, citing domestic concerns about hashish use. The inclusion was somewhat hasty, lacking the scientific review applied to opium and cocaine, but it set the template: cannabis was now formally an international drug control issue.

The 1925 International Pharmacopoeia Agreement further formalized restrictions on medical cannabis. Together, these treaties created the international legal framework that would be strengthened repeatedly over the following decades, culminating in the 1961 Single Convention on Narcotic Drugs—the cornerstone of the modern global drug control regime.

1937: The Marijuana Tax Act

In the United States, the defining domestic moment came in 1937 with the Marijuana Tax Act. Harry Anslinger, the first commissioner of the Federal Bureau of Narcotics, led a campaign of disinformation and racial fear-mongering to push the legislation through Congress. His testimony included fabricated stories of marijuana-induced violence, explicitly linking cannabis use to Black and Mexican communities.

The Tax Act didn't technically criminalize cannabis—it imposed registration and tax requirements so burdensome that legal use became virtually impossible. The American Medical Association actually opposed the legislation, arguing that it would impede legitimate medical research. Congress ignored them.

1970: The Controlled Substances Act

Richard Nixon's Controlled Substances Act placed marijuana in Schedule I—the most restrictive category, alongside heroin and LSD—defined as having high abuse potential, no accepted medical use, and no safe level of use under medical supervision. This classification persisted for 54 years despite growing scientific evidence contradicting every criterion.

Nixon's own commission—the Shafer Commission—recommended decriminalization in 1972. Nixon rejected the recommendation. Decades later, released White House recordings confirmed what many suspected: Nixon's drug war was partially motivated by political targeting of anti-war activists and Black communities.

The War on Drugs: 1980-2010

The Reagan, Bush, and Clinton administrations escalated cannabis enforcement dramatically. Mandatory minimum sentencing, three-strikes laws, and aggressive policing created a mass incarceration crisis that disproportionately affected communities of color. By the peak of enforcement, approximately 800,000 Americans were arrested annually for marijuana offenses—the vast majority for simple possession.

The racial disparities were staggering and well-documented. Despite roughly equal usage rates across racial groups, Black Americans were arrested for marijuana offenses at 3-4 times the rate of white Americans. In some jurisdictions, the disparity exceeded 8:1.

The Unraveling: 2012-Present

Colorado and Washington's legalization votes in 2012 began the cascade. By 2026, 24 states plus Washington D.C. have legalized adult-use cannabis, and 38 states have medical programs. Public opinion shifted dramatically—from roughly 25% support for legalization in the 1990s to over 70% today.

The Schedule III reclassification process, while not full descheduling, represents the most significant federal policy shift in cannabis's legal history. It acknowledges what science has demonstrated for decades: cannabis has accepted medical applications and doesn't belong alongside heroin and LSD.

What a Century of Prohibition Produced

The legacy of 101 years of cannabis prohibition includes millions of criminal records that continue to affect employment, housing, and education opportunities. An estimated 40,000+ Americans remain incarcerated for cannabis offenses even as legal markets generate billions in revenue. Entire communities—predominantly Black and brown—bear the economic and social scars of aggressive enforcement.

The irony is bitter: the same plant that put people in cages now generates tax revenue for the same governments that prosecuted them. Social equity programs attempt to address this injustice, but the scale of repair needed far exceeds what any grant program or licensing preference can deliver.

Looking Forward from Year 101

As we enter the second century of cannabis prohibition—even as that prohibition crumbles—the lessons are worth stating clearly. Policy built on racial fear rather than scientific evidence causes immense human suffering. Criminal justice approaches to public health issues create more problems than they solve. And once prohibition becomes institutionalized, reversing it takes generations.

The arc of cannabis policy is bending toward rationality, but slowly and with tremendous resistance. Full federal legalization remains uncertain in its timeline. International drug treaties still technically require member nations to restrict cannabis. And millions of people around the world still face criminal prosecution for a plant that grows wild on every inhabited continent.

Year 101 of prohibition finds us in an awkward middle ground: legal here, illegal there, rescheduled but not descheduled, celebrated and criminalized simultaneously. The next century of cannabis policy will be written by those who remember what the first century cost—and refuse to accept those costs as inevitable.