In the 1970s a US psychiatrist reported that cannabis caused lasting damage to monkeys’ brains. The claim reached the public in 1974. It entered US politics and was shortened to “cannabis kills brain cells.” No other team ever repeated the result, and two larger monkey studies found no such damage. In the same years, US-funded anthropologists studied heavy long-term users in Jamaica and found little obvious harm. Cannabis advocates have quoted that result ever since. The two stories mirror each other. Together they show how an old result becomes fixed public belief and keeps being repeated after research has moved on.
This part covers the monkey study, what happened to it and why it lasted. Part 2 covers the Jamaica and Costa Rica field studies, five questions to ask of any dramatic finding, and what applies in Thailand.
This article is a case study in how to read research, and news about research. It covers US-funded research from the 1970s and 1980s. It does not claim that cannabis is harmless, and nothing here is medical advice. Current evidence on the risks of cannabis is discussed in the Science and Research section. Practical dosing questions belong in the topic Start Low, Go Slow.
1. The Study and Its Claim
Robert Galbraith Heath was a psychiatrist at Tulane University in the United States. In the 1970s he reported that cannabis caused lasting changes in the brains of rhesus monkeys. In a December 1974 interview with People magazine, he put it this way: “Alcohol is a simple drug with a temporary effect. Marijuana is complex with a persisting effect.” [2]
Two press reports described the experiments. A 1978 report in The New York Times said the monkeys were trained to smoke the equivalent of one cannabis cigarette a day. They smoked five days a week, for six months [3].
The 1974 interview described a study funded by the US National Institutes of Health, the US government’s medical research agency. It used 13 monkeys in three groups [2]:
- heavy exposure: about three joints a day;
- moderate exposure: one joint a day;
- control group: monkeys that smoked inactive cannabis. A control group goes through the same procedure without the active drug, so the researchers can compare it with the exposed groups.
The order of the dates matters. The claim became public in 1974. That same year, the case that cannabis damages the brain was presented at hearings of a US Senate subcommittee chaired by Senator James Eastland [4]. The press and politicians then took it up.
Heath’s main peer-reviewed paper came out six years later, in 1980, in the journal Biological Psychiatry [1]. “Peer-reviewed” means other scientists checked the paper before the journal accepted it. The paper reported two studies covering 21 monkeys in total.
The authors described abnormal recordings of activity from deep brain areas. They also described changes at the synapses, the connections between nerve cells. They saw these changes under an electron microscope. So the public heard the message years before the full paper could be checked.
The public version was also blunter than anything in the papers: “cannabis kills brain cells.” The 1980 paper reported changes in brain structure, not the death of brain cells.
Try this: When you meet a strong claim about cannabis, find the date of the first news report and the date of the scientific paper. If the news came first, read what the paper actually reports before you repeat the claim.
2. What Came Next: No Replication and Two Larger Studies
No other research team ever repeated Heath’s result. Instead, two larger monkey studies followed.
- The National Center for Toxicological Research (NCTR), a US government research centre. A team led by Dr William Slikker used about 64 monkeys. Half were exposed to marijuana smoke for a year. The first results were published in 1991 [5].
- SRI International, a US research institute. Charles Rebert and Gordon Pryor exposed more than 30 monkeys for 6–12 months. They reported their results in 1993 [6].
In 1992, California NORML, a group that campaigns for cannabis law reform, summarised both studies [7]. The monkeys’ brains were examined a few months after exposure ended. Neither study found structural changes or neurochemical changes, meaning changes in brain chemistry.
That is the whole scientific story. There was a striking result. No team repeated it. Two larger studies contradicted it.
Try this: When you check a claim, look for later studies on the same question, not only the first one. Searching for the original author’s name together with the topic and a later decade is a simple way to start.
3. Replication: Why One Study Is Not Enough
Replication means that a different, independent team repeats an experiment and checks whether it gets the same result. Until that happens, a single study is a hypothesis, an idea that still needs testing. This holds however striking the result is. Replication is what turns a result into knowledge.
One study can mislead for several reasons. With few animals or people, chance alone can produce a large difference. The method may contain a flaw that the original team did not notice. Conditions in one laboratory may differ from everywhere else.
This is why the repeat has to come from an independent team, not the original lab. An independent team uses its own equipment and its own staff. It also has no reason to want the first result to be true.
Sample size matters too. A small study can raise a question, but it rarely settles one on its own. Heath’s work used 13 monkeys in the study described in 1974, and 21 in the 1980 paper. The two later studies came from independent teams and used about 64 and more than 30 monkeys. Neither found brain damage, so the evidence moved against Heath’s claim.
The same logic works on a farm. Say one tent gives a bigger harvest after you add a new product. The product could be the cause. A cooler month, a different clone or a better batch of substrate could also be the cause. You would repeat the test on the next run before changing the whole farm. It is even better if another grower tries it too and gets the same result.
Try this: Before you act on a striking result, ask one question: has an independent team repeated it? If you cannot find a repeat, treat the result as an open question, not a fact.
4. The “Suffocated Monkeys” Charge: Disputed, Not Proven
Pro-cannabis writing makes a much stronger accusation. It says the monkeys were not smoking in any normal sense. According to this account, they were pumped with large amounts of smoke through a mask over a few minutes, without enough oxygen. The damage would then come from carbon monoxide poisoning and lack of oxygen. It would not come from the cannabinoids, the active compounds in cannabis.
The charge is usually traced to 1980. That year, details of how Heath exposed the monkeys were released and reported by Playboy magazine. Advocacy accounts say the release followed requests under US freedom-of-information law. That law lets the public request government documents.
This charge is disputed, not proven. It rests on one reading of the exposure method. No team has published a repeat of the experiment that confirms it, and the journal has not retracted (withdrawn) the paper. The original Playboy report and the released documents could not be checked for this article.
The record that can be checked says something simpler. Heath’s study lost credibility because no one could replicate it. That record does not name a specific flaw that produced a false result.
This difference matters, and it is a reason to treat the suffocation story with caution. You do not need it to reject Heath’s claim. The failed replication and the two larger contradicting studies are enough on their own, and both are documented independently. Presenting the dramatic version as fact is the same mistake this article describes. This time the pro-cannabis side makes it.
Try this: When you correct a myth, use the reason you can document. If you have a more dramatic explanation but cannot check its source, say so clearly or leave it out.
5. Why the Claim Survived Anyway
Four forces kept the claim alive, and all four still operate today.
Politics. The finding entered the political speeches of the time. Many later accounts say this included Ronald Reagan, then Governor of California. Anti-drug education repeated the claim for years. Once a claim becomes part of a political programme, it keeps its supporters whether or not the evidence holds up.
Unequal news coverage. “Cannabis kills brain cells” makes a headline. “A 1970s monkey study was not repeated” does not. A failed replication gets only a small share of the attention that the first claim received.
You can still measure this pattern today. In 2022, Gedin and colleagues published a meta-analysis in the journal JAMA Network Open [8]. A meta-analysis combines the results of many studies on one question to get a more reliable overall answer.
This one combined randomised trials of cannabis-based treatments for pain. In a randomised trial, chance decides who gets the treatment and who gets a placebo. A placebo is a dummy treatment that looks the same as the real one. Overall, pain relief with cannabinoids did not differ significantly from placebo. Even so, media attention was high and mostly positive, whatever each trial had found.
Old material keeps circulating. Once a school or an agency prints and hands out education material, that material stays in use. Teachers who learned the claim go on to teach it.
It matched what people already believed. Claims last longest when an audience is ready to accept them. That works in both directions. Part 2 shows the same pattern on the pro-cannabis side, with old field studies from Jamaica and Costa Rica.
Try this: When a post or news story reports a dramatic cannabis finding, look for what came next. Search for later studies, or for a meta-analysis on the same question. Judge the claim by that follow-up, not by the headline.
Translator’s note: The same four forces work on growing advice in Thailand and Southeast Asia. A striking result about a nutrient, an additive or a lighting change spreads fast through shops, social media and group chats. A later test that finds no effect rarely spreads as far. Many trials also take place in cooler climates or in controlled rooms. Their results may not hold at 30–35 °C, in rainy-season humidity, or on a farm with frequent power cuts.
Translator’s note: Before you change the whole farm, test the change on a small batch for at least one full run. Grow an unchanged batch beside it as your control. Record temperature, humidity, feed and yield for both batches. This shows you whether the result holds under your own conditions, not just in someone else’s trial.
You can ask questions or share your own test results in the replies below this topic.
References
- Heath RG, Fitzjarrell AT, Fontana CJ, Garey RE. Cannabis sativa: effects on brain function and ultrastructure in rhesus monkeys. Biological Psychiatry. 1980;15(5).
- Chandler D. “Pot Is Safe, Right? Wrong, Says a Doctor: It Can Cause Brain Damage.” People, 9 December 1974. https://people.com/archive/pot-is-safe-right-wrong-says-a-doctor-it-can-cause-brain-damage-vol-2-no-24/
- Rensberger B. “Marijuana Tied to Brain Change in Monkey Tests.” The New York Times, 13 January 1978. https://www.nytimes.com/1978/01/13/archives/marijuana-tied-to-brain-change-in-monkey-tests.html
- US Senate, Committee on the Judiciary, Subcommittee to Investigate the Administration of the Internal Security Act. Marihuana-Hashish Epidemic and Its Impact on United States Security. Hearings, 93rd Congress, 2nd session – 94th Congress, 1st session (9 May 1974 – 8 May 1975). https://catalog.hathitrust.org/Record/001278569
- Slikker W Jr, Paule MG, Ali SF, Scallet AC, Bailey JR. Chronic marijuana smoke exposure in the rhesus monkey. I. Plasma cannabinoid and blood carboxyhemoglobin concentrations and clinical chemistry parameters. Fundamental and Applied Toxicology. 1991;17(2):321–334. doi:10.1016/0272-0590(91)90222-p
- Rebert CS, Pryor GT. Chronic inhalation of marihuana smoke and brain electrophysiology of rhesus monkeys. International Journal of Psychophysiology. 1993;14(2):144 (abstract). doi:10.1016/0167-8760(93)90230-m
- California NORML Reports, April 1992 (reprinted by Erowid). https://www.erowid.org/plants/cannabis/cannabis_myth16.shtml
- Gedin F, Blomé S, Pontén M, et al. Placebo response and media attention in randomized clinical trials assessing cannabis-based therapies for pain: a systematic review and meta-analysis. JAMA Network Open. 2022;5(11):e2243848. pubmed.ncbi.nlm.nih.gov
General educational information on evaluating scientific claims. Not medical advice.
Continued in Part 2: The Dead Monkey Study: The Jamaica and Costa Rica Field Studies, Five Questions for Any Finding, and What Applies in Thailand (Part 2 of 2)