In the early 1970s, US-funded researchers studied men in Jamaica who had used cannabis heavily for most of their lives. A separate team later studied long-term users in Costa Rica. Neither study found gross medical or psychiatric harm, and cannabis advocates have quoted both ever since. Later research with better tools found more specific harms in more specific groups. This part covers what the field studies showed and what they could not show. It also gives five questions to ask of any dramatic finding, and explains what local evidence and Thai law add.
Part 1 told the other half of the story. In the 1970s, the US psychiatrist Robert Galbraith Heath reported that cannabis caused lasting brain damage in monkeys. The public heard it as “cannabis kills brain cells.” No other team repeated his result, and two larger monkey studies found no structural brain changes. The Jamaica story is the mirror image. An old result suits the other side, and people still quote it after research has moved on.
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.
This is Part 2. Part 1: The Dead Monkey Study: How One Unreplicated Experiment from the 1970s Shaped Beliefs About Cannabis and the Brain, and the Jamaica Study That Mirrors It
1. The Field Studies in Jamaica and Costa Rica
The Jamaica study is a book by the anthropologists Rubin and Comitas: Ganja in Jamaica: A Medical Anthropological Study of Chronic Marihuana Use (1975) [1]. Anthropologists study how people live in their own communities. “Ganja” is the local word for cannabis. The Research Institute for the Study of Man carried out the study. The US National Institute of Mental Health, a US government agency, funded it.
The fieldwork took place in the early 1970s, roughly 1970–72. The book came out in 1975.
The Costa Rica study was a separate project. A team led by William E. Carter, an anthropologist at the University of Florida, carried it out. The team published it as Cannabis in Costa Rica: A Study of Chronic Marihuana Use (1980) [3].
Both studies used a design that is still their main strength. The researchers did not bring volunteers into a laboratory. They studied long-term, heavy users in their own communities, where cannabis was part of local culture. They combined medical examinations with ethnographic fieldwork, meaning long-term observation of daily life.
This gave them real-world exposure over many years, which a single-dose laboratory study cannot provide. Their main finding was reassuring. They found no gross medical or psychiatric harm caused by long-term use.
Advocacy writing often adds broader claims. One is that there was no “gateway effect”, the idea that cannabis leads users on to other drugs. Another is that cannabis replaced rum in Costa Rica. These broader claims stand on weaker ground than the main finding.
Try this: When someone cites these studies, check which claim they mean. Is it the main finding, no gross harm in that group? Or is it one of the broader claims?
2. The Limits That Are Rarely Mentioned
People who cite these studies almost never mention the following limits.
- Small samples and no random assignment. The Jamaican clinical comparison involved 30 long-term users and 30 matched controls [2]. Matched controls are non-users chosen to resemble the users. The Costa Rican clinical work also compared a small number of matched pairs of users and non-users. Neither study assigned people at random to use or not use cannabis.
- What observational studies can and cannot show. In an observational study, researchers watch and measure without deciding who uses the drug. This design can describe a population. It cannot show what would have happened to the same people without cannabis.
- Healthy-user and survivor effects. The researchers could only study people who were still working, still living in their communities and still willing to take part. People whose lives had gone badly were less likely to be in the sample. So the sample looked healthier than all long-term users really were. The same filter makes personal testimony unreliable.
- The product was different. Jamaican ganja around 1970 is not the high-potency cannabis of 2026. A European study called EU-GEI compared people with a psychotic disorder and people without one [6]. A psychotic disorder is a serious mental illness in which a person loses touch with reality. Comparing people with and without an illness in this way is called a case-control study.
- What EU-GEI found. Daily cannabis use was linked to roughly three times the odds of psychotic disorder, compared with never using. Daily use of high-potency cannabis, meaning THC ≥10%, was linked to roughly five times the odds [6].
- Cruder memory and thinking tests. The tests available around 1970 were less sensitive than those used today.
- A published critique. In 1985 the researcher G. G. Nahas published a formal critique of the Jamaica study. It appeared in the Bulletin on Narcotics, a United Nations journal [4]. It focused on the study’s methods and its small number of subjects. If you cite the Jamaica findings without this debate, the evidence looks more settled than it is.
Try this: If you cite the Jamaica or Costa Rica studies, state the sample size and the era in the same sentence. Then mention the 1985 critique as well.
3. What Later Research Found
The evidence has grown since the 1970s. It has become more detailed, not more favourable to cannabis.
- Memory and attention. A 1996 study looked again at long-term users in Costa Rica (Fletcher and colleagues). Older long-term users did worse than non-users on tests of short-term memory and attention [5]. The earlier field methods were not designed to detect this kind of specific deficit.
- Withdrawal. Cannabis withdrawal syndrome is a set of unpleasant symptoms that can appear when a regular user stops. A 2020 meta-analysis (Bahji and colleagues) combined many studies. It found the syndrome in about 47% of regular or dependent users [7].
- Potency and psychosis. The EU-GEI link between high-potency cannabis and psychotic disorder, described above, is another example.
- The gateway question. In 2017 the US National Academies of Sciences, Engineering, and Medicine published a major review [8]. This body produces expert reviews of scientific evidence. It found moderate evidence that cannabis use is statistically linked to later dependence on other substances. It did not show that cannabis causes this.
- Crime. Researchers still debate the link between cannabis and crime. How much of the link comes from the drug itself is unresolved. How much comes from prohibition and shared social risk factors is also unresolved.
So the honest summary is neither “the Jamaica study proved cannabis is safe” nor “the Jamaica study has been refuted.” Careful observational studies of specific groups in a specific era found no gross harms. Later research with better tools found more specific harms in more specific groups. Both statements are true, and they do not contradict each other.
What still stands. The design was right. Studying real exposure in real settings is a valid method, and researchers use it too little. The studies did not find the catastrophic harm that the political speeches of the time predicted. That is not the same as showing that heavy daily use is safe.
The substitution question. People often credit the Costa Rica work with this question: do people use cannabis in place of alcohol? Some surveys support it. No randomised trial has tested it, so it remains a hypothesis. The topic Cannabis as Substitution for Alcohol and Opioids covers it in more detail.
The two mistakes are the same mistake. Citing 1975 findings as if nothing had been published since is the mirror image of keeping the dead-monkey claim alive. A related mistake is to apply findings from adult men in rural Jamaica around 1970 to adolescents in 2026.
The Jamaican men had used low-potency cannabis all their lives, under community rules. Today’s adolescents use high-potency products in unregulated markets. The two groups differ on exactly the factors that later research showed to matter. Both sides need to read the whole record up to today, not just the decade that suits them.
Try this: Before you quote an old study, check two things. Has newer research on the same question been published? Do the people in the study resemble the people you are talking about?
4. Five Questions to Ask of Any Dramatic Finding
These questions apply to every dramatic finding, including the ones you would like to be true.
| Ask | Why |
|---|---|
| Has it been replicated? | A single study is a hypothesis, however striking. Replication is what turns it into knowledge |
| By whom? | The repeat should come from independent groups, not the original lab |
| What was the actual exposure? | Dose, route and duration decide whether the experiment resembles real-life use |
| What is the sample size? | Small samples can raise a question, but they rarely settle one alone |
| What happened next? | For a 1970s claim, the most useful fact is what researchers did with it over the next fifty years |
The last question is the most useful, and people ask it least often. The epilepsy story went the opposite way from Heath’s. In 1980, Cunha and colleagues ran a small trial of cannabidiol (CBD) in healthy volunteers and patients with epilepsy [9]. Researchers kept following up this result until cannabidiol became a licensed medicine. Comparing the two cases is the clearest way to see the difference between a result that holds up and one that does not.
Try this: Copy these five questions into your notes. Use them the next time a supplier, a forum post or a news story cites a study, and write down the answers.
5. A Balanced Position
Heath’s specific claim does not stand. No one has shown that cannabis kills brain cells in the way he described.
That does not mean cannabis has no effect on the brain. Thinking and memory are clearly impaired while a person is intoxicated [8]. In the Costa Rica follow-up, older long-term users scored worse on memory and attention [5]. Frequent use of high-potency cannabis is linked to higher odds of psychotic disorder [6]. All of this rests on larger and better evidence than Heath ever had.
The Jamaica findings do not support the opposite conclusion either. They describe one population, one era and one product.
The right lesson from both episodes is not “cannabis is safe” or “cannabis is dangerous.” The lesson is that one study is not proof, whichever way it points. Decades of argument over an old animal study or an old field study cannot replace larger, better-designed studies in humans. Only those studies can actually answer the question.
Try this: If you share either story, share its limits with it. Heath’s claim was never replicated. The Jamaica findings cover one population, one era and one product.
6. Why This Still Matters in Thailand and Southeast Asia
Both stories are American, but the message they created outlived them. Versions of “cannabis kills brain cells” still appear in drug-prevention and public-information material in many countries. Parts of this region may be among them. One likely reason is that prevention messages favour simple, memorable warnings. Simple claims tend to outlast the evidence behind them.
This article makes no claim about any specific national curriculum. If you have seen this claim in local materials, post where you saw it.
Thailand also has its own, more relevant evidence. Cannabis was removed from the narcotics list in 2022. Since then, Thai researchers have published data on the period when cannabis was widely available. This includes nationally representative surveys of adult cannabis use [10]. Researchers design these surveys so that the people sampled reflect the adult population of the whole country.
The current legal position. Cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. The notification was issued under the Thai Traditional Medicine Protection Act B.E. 2542 (1999) and has been in force since 26 June 2025.
Flower may be sold only against a PT 33 (ภ.ท.33) prescription from an authorised practitioner. Each prescription covers at most 30 days.
Any flower sold must come from a site certified under GACP (Good Agricultural and Collection Practice), a standard for how medicinal plants are grown, harvested and recorded. Advertising and online sale of cannabis flower are prohibited.
The draft Cannabis Control Act is not law yet. Cabinet approved the draft on 22 September 2026 and sent it to the House.
Translator’s note: As a grower or a shop worker, you may hear customers or visitors repeat the dead-monkey claim or the “Jamaica proved it is safe” claim. Do not answer with a health claim in either direction. Refer health questions to a licensed practitioner, since flower is sold only on a PT 33 prescription anyway. Point to the evidence topics on this forum. This keeps the conversation factual and keeps you away from medical claims you cannot support.
Translator’s note: Apply the five questions to growing claims too. Examples include an additive that “boosts yield”, a strain said to “resist mould” or a lighting change said to “raise potency”. Ask for the source and check whether anyone has repeated the result. Then test the claim on a small batch beside an unchanged batch under your own heat and humidity before you scale up.
Share what you know. If you know of Thai or regional research on cannabis use and outcomes since 2022, post it in the Science and Research section with the full reference. If someone taught you the dead-monkey claim, or another cannabis “fact” you later found to be wrong, post where you heard it.
Studies from other countries cannot answer questions about people in this region, but local research can. Tracing where a piece of received knowledge came from is a skill. You can use it on every other health claim you meet. You can also ask questions or start the discussion directly below this topic.
References
- Rubin V, Comitas L. Ganja in Jamaica: A Medical Anthropological Study of Chronic Marihuana Use. The Hague: Mouton; 1975.
- Beaubrun MH, Knight F. Psychiatric assessment of 30 chronic users of cannabis and 30 matched controls. American Journal of Psychiatry. 1973;130(3):309. https://doi.org/10.1176/ajp.130.3.309
- Carter WE, ed. Cannabis in Costa Rica: A Study of Chronic Marihuana Use. Philadelphia: Institute for the Study of Human Issues; 1980.
- Nahas GG. Critique of a study on ganja in Jamaica. Bulletin on Narcotics. 1985;37(4):15–29. https://ojp.gov/pdffiles1/Digitization/119741NCJRS.pdf
- Fletcher JM, Page JB, Francis DJ, et al. Cognitive correlates of long-term cannabis use in Costa Rican men. Archives of General Psychiatry. 1996;53(11):1051–1057. https://scholarworks.gsu.edu/psych_facpub/169
- Di Forti M, et al. The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI). Lancet Psychiatry. 2019;6(5):427–436.
- Bahji A, et al. Prevalence of cannabis withdrawal symptoms among people with regular or dependent use of cannabinoids: a systematic review and meta-analysis. JAMA Network Open. 2020;3(4):e202370.
- National Academies of Sciences, Engineering, and Medicine. The Health Effects of Cannabis and Cannabinoids. Washington, DC: National Academies Press; 2017.
- Cunha JM, et al. Chronic administration of cannabidiol to healthy volunteers and epileptic patients. Pharmacology. 1980;21(3):175–185.
- Wichaidit W, et al. Density of cannabis outlets vs. cannabis use behaviors and prevalent cannabis use disorder: findings from a nationally-representative survey. PeerJ. 2024;12:e17317. Density of cannabis outlets vs. cannabis use behaviors and prevalent cannabis use disorder: findings from a nationally-representative survey - PMC
General educational information on evaluating scientific claims. Not medical advice.