The history of cannabis regulation in the United States is not a straight line from prohibition to reform. Cannabis was a legal, commercially available medicine before it was banned, and the ban arrived through a mechanism that never quite said what it was doing.
Cannabis as a Listed Medicine
Cannabis entered Western medicine substantially through William Brooke O’Shaughnessy, an Irish physician working in Calcutta in the 1830s. He studied its use in Indian medicine, conducted his own trials, and published on its application to conditions including rheumatism, tetanus and convulsions — including a case of infantile convulsions that drew considerable attention.
His work brought cannabis into British and then American medical practice. By the second half of the nineteenth century it was a standard pharmacy item, listed in the United States Pharmacopeia and sold by major pharmaceutical manufacturers as tinctures and extracts for pain, sleep, appetite and muscle spasm.
Its decline in medical use began before any legal restriction. Injectable opiates and, later, aspirin offered more predictable dosing — cannabis preparations varied enormously in strength, which is precisely the problem that potency testing exists to solve.
The 1937 Marihuana Tax Act
The federal turn came in 1937. The Marihuana Tax Act did not formally prohibit cannabis — it imposed a tax and a registration requirement on anyone dealing in it, with penalties for non-compliance severe enough that the practical effect was prohibition.
The legislative record around it is not edifying. The campaign leading to the Act relied heavily on sensational press coverage, and the terminology itself — “marihuana” rather than the cannabis familiar to physicians and pharmacists — did work that was partly deliberate. Racial politics of the period were explicitly present in the advocacy.
The American Medical Association opposed the Act, objecting both to the restriction on a medicine still in use and to the process by which it was passed.
Cannabis was removed from the United States Pharmacopeia in 1942. Within five years it went from a listed medicine to a substance most physicians could not practically prescribe.
The Controlled Substances Act
The Marihuana Tax Act was struck down in 1969 on constitutional grounds — the registration requirement compelled self-incrimination. Congress responded with the Controlled Substances Act of 1970, which placed cannabis in Schedule I: the category reserved for substances deemed to have high abuse potential and no accepted medical use.
Schedule I is the foundation of everything since. It is why cannabis research has been difficult to conduct, why federal agencies have no framework for regulating cannabis products, why banks avoid the sector, and why state programs exist in tension with federal law rather than under it. Practical consequences are covered in why dispensaries still operate in cash.
States Move First
Reform came from the states, and it came slowly.
California’s Proposition 215 in 1996 was the first medical cannabis program in the modern era. Over the following two decades a majority of states followed with medical frameworks, and from 2012 onward a growing number added adult-use legalization.
Each state built its own system — its own eligibility rules, licensing structure, product standards and testing requirements — because there was no federal framework to build on. That is the direct cause of the patchwork described in how testing rules differ across states.
The Case That Changed the Conversation
Public opinion shifted for many reasons, but one case did disproportionate work.
Charlotte Figi was a Colorado child with Dravet syndrome, a severe treatment-resistant epilepsy. Her family’s use of a high-CBD, low-THC cannabis extract, and the reported reduction in her seizures, received national coverage in 2013. The strain was renamed Charlotte’s Web.
The effect on policy was rapid and specific: a wave of states that had shown no interest in broad medical cannabis passed narrow CBD-only laws. It also refocused attention on CBD as distinct from THC and, importantly, on product consistency — families were dosing a child with an unregulated preparation, and that made composition testing a mainstream concern rather than a technical one. The fuller account is in Charlotte Figi and the turning point for CBD.
Hemp Splits Off
The 2018 Farm Bill removed hemp — cannabis containing no more than 0.3% delta-9 THC by dry weight — from the Controlled Substances Act.
That created two parallel legal categories for what is botanically one plant, distinguished by a threshold. It also created the CBD retail market and, less intentionally, a market in hemp-derived intoxicating cannabinoids that the drafters did not anticipate.
Missouri’s Route
Missouri legalized medical cannabis by ballot initiative in November 2018 — Amendment 2 — and wrote the framework into the state constitution rather than leaving it to the legislature. That framework was capped from the start: no more than twenty-four dispensary licenses per congressional district, with minimum cultivation and manufacturing counts tied to population. Testing facilities were deliberately left uncapped.
Amendment 3 followed in November 2022, adding adult use and converting existing medical licensees, with the first adult-use sales in February 2023. Testing rules were codified at 19 CSR 100-1.110, which requires laboratories to hold ISO/IEC 17025 accreditation within a year of approval and to pass annual proficiency testing.
The tightening since has been about ownership and supply rather than access. Microbusiness rules were revised in 2026 after licenses were revoked over ownership arrangements, and legislation signed in April 2026 reclassified intoxicating hemp products as marijuana, restricting them to licensed dispensaries from November 2026.
That arc — a framework written at the ballot box, expanded by a second vote, then tightened around ownership and product scope — is a common pattern across states that legalized by initiative.
What the History Suggests
Two things stand out.
First, cannabis was regulated out of medicine largely for reasons unrelated to evidence, and rebuilding an evidence base has taken decades — partly because Schedule I made the research itself difficult. That is why the state of the evidence is patchier than the length of the plant’s medical history would suggest. See the science behind medical cannabis research.
Second, the nineteenth-century problem and the modern one are the same problem. Cannabis preparations varied unpredictably in strength, which limited their clinical usefulness then and drives the testing requirements now. The difference is that we can now measure what is in a product. That capability is what separates a regulated cannabis market from an unregulated one — as much as any statute.