That's the honest question underneath "was the Constitution written on hemp" once you keep pulling the thread. Cannabis grew here before there was a United States. Then, in one specific decade, it became a federal crime — on the strength of a campaign, not a discovery. Here's the record of how that happened, and what the government's own later research found when it finally checked the story.
Hemp was a legal, common colonial-era crop grown for rope and sailcloth; that's settled history, and it's a completely separate fact from what the signed Constitution is physically made of (parchment — see the last entry if you missed it). Being a real, useful, legal plant for most of this country's history is also a separate fact from why it became illegal. That second question is the one worth answering, because the answer isn't "science found it was dangerous." The answer is a specific campaign, run by a specific person, in a specific decade.
In 1937, Congress passed the Marihuana Tax Act — the first federal regulation on cannabis in U.S. history. It was drafted and pushed hard by Harry J. Anslinger, head of the Federal Bureau of Narcotics, who told Congress the Bureau had seen a rise in marijuana use and linked it to addiction, violence, and overdose.
The American Medical Association testified against the bill. Its own legislative counsel, Dr. William Creighton Woodward, told Congress directly that the claims about marijuana causing addiction, violence, and overdose were not supported by evidence. Congress passed the act anyway.
The Marihuana Tax Act became federal law in 1937, effective October 1. Confirmed
The Federal Bureau of Narcotics' own head drafted and championed it. Confirmed
The addiction/violence/overdose claims used to justify it were contested at the time by the AMA's own legislative counsel — and not supported. Documented in the congressional record itself
The 1936 film now known as Reefer Madness (originally released under other titles) depicted teenagers descending into hallucination, manslaughter, and suicide after a single use of marijuana. It was a dramatized film, not a documentary — produced in the same climate Anslinger's campaign was building, and its claims don't hold up as medicine or fact. It became a cult joke decades later precisely because how far it strayed from reality is now obvious to almost everyone who watches it.
The pattern to notice: a scare campaign and a federal law arrived in the same stretch of years, telling the same story, and the story wasn't true when independent experts checked it at the time. That's not a modern reassessment — the AMA said so in 1937, while the vote was still happening.
D.A.R.E. (Drug Abuse Resistance Education) started in Los Angeles in 1983, built with the LAPD, and sent uniformed police officers into elementary classrooms nationwide to teach kids to refuse drugs. It became the most widely used school drug-prevention program in the country during the "Just Say No" era. Then researchers actually measured whether it worked.
A National Institute of Justice review (NCJ 152055) concluded: "The program imparts a large amount of information but appears to have little or no impact on drug use."
The U.S. Surgeon General's report Youth Violence, under "Ineffective Primary Prevention Programs," lists D.A.R.E. as the one school-based universal program meeting the criteria for "Does Not Work."
The U.S. General Accounting Office (GAO-03-172R, January 15, 2003): "All of the evaluations suggested that DARE had no statistically significant long-term effect on preventing youth illicit drug use."
None of that means the researchers thought kids should use drugs. It means the specific program — taught in good faith to a whole generation, this site's author included — was checked against reality by the government's own agencies, and it didn't hold up as prevention. D.A.R.E. later introduced a revised, researcher-designed curriculum, keepin' it REAL, starting in 2009 (Scientific American).
A whole generation was taught, sincerely, by trusted adults, using government-backed curriculum, a story that the government's own later research didn't support. That's a hard thing to sit with, and it's also not unique to this one topic — it's why this site keeps asking "what else isn't true to the record" instead of assuming the first answer we were given was the last word. Nebraska voters said yes to medical cannabis by a wide margin; that vote, and what happened to it afterward, is its own page.