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Home » Marijuana and cancer – what are facts and what’s just smoke

Marijuana and cancer – what are facts and what’s just smoke

Last updated on September 14th, 2020 at 12:03 pm

Editor’s note–this article has been updated and included into a multi-part series on marijuana and medicine. Check it out there. 

If you spend any amount of time on Twitter, Facebook, or just researching cancer treatments on the internet, you will run across something about marijuana and cancer – someone will claim that smoking pot, eating pot, hemp oil (which is manufactured from the seeds of cannabis plants that don’t contain much THC, or tetrahydrocannabinol, the active hallucinogenic agent of cannabis), or some other consumption of cannabis will cure or prevent cancer.

Of one hand, studies of cannabis’ effectiveness in reducing nausea and vomiting in chemotherapy, one of the the most common claims,  has generated negative results in some well done clinical trials and some positive results in others. But that has nothing to do with actually curing or preventing the cancer itself, just dealing with the effects of the treatment.

Scientific evidence for marijuana and cancer


So, is there any evidence out there that actual cannabis or its byproducts have any effect on cancers? Before we start, let’s remember that there are 100 to over 200 different types of cancer (the actual number depends on how some researchers subdivide some types) in humans. And each of these different cancers have different pathophysiologies, different genetics, different prognoses, different causes, and different treatments. In other words, it is not one singular disease with one unified course of treatment.

Always be skeptical when someone makes some claim that “XYZ cures or prevents cancer”, because that’s going to be nearly impossible. Every cancer is so different with such different physiology, there is just never going to be a magic pill.

So before I head down the the road of whether THC or cannabis has any effect on any particular, I’m not going to play the argument from ignorance. If there isn’t any evidence that pot cures a particular cancer, then that means there’s no evidence. I’m not going to think “oh, they just haven’t tested it on THAT cancer, so how can you not be sure that pot doesn’t cure THAT cancer.” I also don’t have evidence there’s a pink unicorn walking on Mars, but I’m fairly certain of it.

The first step I take before I investigate any internet claim is check if there are  systematic reviews of randomized clinical research to find the best information for evidence based medicine.

A search of the Cochrane Reviews, one of the best sources of systematic reviews in clinical medicine, shows not one systematic review of THC or cannabis in cancer therapies (although several in some mental health issues, which may be a topic for some other article). There are reasons why Cochrane may not have a review can be many, but mainly it’s because there just aren’t enough studies of high enough quality to roll up into a systematic review. That’s a clue, but it’s more a lack of evidence rather than solid evidence that cannabis has no effect on cancer.

Look at it another way–there are currently over 8000 clinical trials that are recruiting patients for anti-cancer drugs (and over 19000 that are closed to patients). Cannabis research is a tiny speck (though not an insignificant speck) of the cancer research world, compared to the vast body of research currently ongoing.

Cancer research isn’t randomly throwing compounds at cells and seeing if they work or not, it’s a logical process to determine if a compound has a reasonable chance of inhibiting some part of the cancer growth or development. There is some potential for cannabis, but it probably doesn’t compare to what is currently in the 27,000 compounds in current clinical research, and, in a business sense, the return on investment for researching compounds that have better understood pharmacologic mechanisms of action are higher.



Many claims about marijuana and cancer


Let’s see what’s going on recently. Remember, if some article was published 10 years ago, and there’s not one single follow-up study, it’s dead on the vine, meaning that no one was able to repeat the data. No one was able to move it from some cell-culture or animal model to a human clinical trial. This happens all of the time, because failure in oncology drugs is the norm.

  • Cannabinoids: A new hope for breast cancer. “This review summarizes our current knowledge on the anti-tumor potential of cannabinoids in breast cancer, which suggests that cannabinoid-based medicines may be useful for the treatment of most breast tumor subtypes.” Now, you might think that smoking a joint is going to cure breast cancer. No, it won’t. The initial studies are done in cell culture, not in human clinical trials. Furthermore, the active ingredient must be delivered in the correct dosage over the correct amount of time at the actual tumor site to cause death of the cancer cells. This type of research will take years, even if it is successful. Smoking or consuming pot would never produce a THC level high enough at the breast cancer site to kill the cancer death. In fact, to do so might be harmful to the patient, so, as they say, don’t try this at home.
  • Chemopreventive effect of the non-psychotropic phytocannabinoid cannabidiol on experimental colon cancer. The authors concluded that  cannabidiol, a non psychotropic ingredient of cannabis, can exert a chemopreventive effect on colon cells in cell culture. But once again, this has been shown in humans and has not shown whether there is a dose response that works. So, eating a bunch of cannabis is not going to work, and the amount necessary to match the cell culture levels would probably be toxic.
  • Hash Oil for Gliomas? What Would You Do? The authors found little credible evidence, though they found studies that showed spontaneous regression of gliomas. Except spontaneous regression of gliomas is not unknown, and fairly common. There was some preliminary evidence that it might work, but hardly a broad based clinical trial that would indicate an oncologist should use hash oil as his first, second, third, fourth or fifth line of treatment for a glioma. As the author stated, “I didn’t find anything I would call “earth-shattering” or even anything that could be considered credible evidence that hash oil can cure advanced gliomas.”
  • Cannabis use and risk of lung cancer: a case-control study. “In conclusion, the results of the present study indicate that long-term cannabis use increases the risk of lung cancer in young adults.” More evidence that smoking marijuana greatly increases risk of lung cancer. To be fair, the evidence is conflicting, but there appears to be increased risks for some types of cancer amongst marijuana smokers.
  • Cannabis and cannaboids anti-tumor effects from the National Cancer Institute. There might appear to be a lot of research into cannabis using cell culture and animal models, but none have been transferred over to human clinical trials. This is not unusual, because even though it seems that there is a lot of research into cannabis and cancer, the total mass of research into other compounds with respect to cancer is substantially larger, because the evidence for both mechanisms of the treatment and clinical successes for these other products are much higher.
  • As of today, there has been only one clinical trial anywhere in the world that attempted to show an effect of any cannabis or cannabis products on cancer. It was a Phase I clinical trial, published six years ago, and has not gone any further. A review of PubMed and show no other clinical trials underway studying cannabis on any type of cancers.
  • A four year old study/editorial published in a top journal concluded that “cannabis-like compounds offer therapeutic potential for the treatment of breast, prostate and bone cancer in patients. Further basic research on anti-cancer properties of cannabinoids as well as clinical trials of cannabinoid therapeutic efficacy in breast, prostate and bone cancer is therefore warranted.” Now, their conclusion was based on preliminary evidence in vivo and in vitro studies, not randomized human clinical trials. Moreover, I cannot find any publication over the past four years that seem to support the ideas proposed by this study. It seems to be another dead end.

Marijuana and cancer–simple math


Let’s say that we actually can gather evidence that marijuana has an effect on breast cancer. First, we need to determine how much THC actually would kill most breast cancer cells. In one study, the researchers determined that it would take a concentration of cannabinoids of approximately 10 µmol/L to cause the death breast cancer cells in cell culture.

This converts to around 3.14mg/L of THC. So, you’d have to assume that to kill any breast cancer cells, you’d need at least a blood level of 3.14 mg/L to cause breast cancer cell death.

So how close to that 3.14 mg/L can we get by just smoking a joint or two? According to research, smoking one cigarette will give you a blood level of THC of around 1.3-6.4 ng/mL serum, or about .00013-.00064 mg/L. In other words, to get the anti-cancer effect that is described in the breast cancer study, you would need to light up around 1000 marijuana cigarettes per day.

Yes, you would need to smoke over 1000 cannabis joints every single day until every single breast cancer cell would die. Of course, your lungs couldn’t tolerate that, nor probably your ability to function in any “normal” manner.

Of course, you could consume this in other ways, for example ingesting it, but again, you’d need to eat more than 1000 joints (because less digestion is less efficient in absorbing THC than the lungs, which is why it is smoked). And these levels may be more generally toxic to body, in effect killing you. If we ever uncover clinical evidence that marijuana “cures” breast cancer, Big Pharma will spend the money researching which molecule is actually responsible for killing the cancer cell, the overall toxic dose where the cannabis (or active ingredient) which harms the body more than cancer cells, the amount necessary to kill the cancer cells, and how to deliver it effectively to the breast cancer cell.

And yes, Big Pharma will patent it, because they did all the hard work, and they will get $10,000 a dose (just a guess). So, let’s be clear. Smoking a couple of joints is NOT going to cure or prevent breast cancer.


Big Pharma and cannabis


I have heard the arguments that Big Pharma, the FDA, the National Cancer Institute (if the cannabis supporters even know it exists), and the US Government suppress all the research that show how great cannabis is for cancer because those groups don’t want marijuana to be legal. As amusing as that ad hominem argument might be (and it’s a fairly amusing one), if cannabis or any of its components actually could show efficacy against any of the 200 or so cancers, Big Pharma would be all over it.

Big Pharma would not be selling individual cannabis cigarettes, they would be isolating the active ingredient from the plant (or parts of the plant), determining the exact dose that would have an exact effect, determining how to deliver precisely to the cancer site, funding clinical trials, filing documentation with the FDA, then getting it into physician’s hands.

This is not an easy process (notwithstanding the $1 billion investment for the whole process), but it would probably be a profitable one if it worked. Big Pharma and the National Cancer Institute are looking at everything to treat all 200 cancers, and they ignore nothing for potential. If cannabis works (and it might), they will be all over it. Big Pharma is providing a lot of the funding for it.

Conclusion, the TL;DR version


  • So, does marijuana  prevent or cure cancer? There is little evidence that it prevents cancer and a little evidence that it can cure cancer. But these are very limited in vivo and animal studies, very preliminary and not in controlled clinical trials. Just to give a little perspective, less than 8% (pdf) of oncology drugs that enter clinical trials actually end up being approved for use in humans.
  • Maybe in time, cannabis will have its place in the armamentarium of anti-cancer drugs if the pre-clinical studies meet the standards of scientific research and are repeated. The science should also identify an appropriate mechanism and what the active ingredient might be. In time, if it can be shown in clinical trials and be approved by the FDA, then it can be important. But right now, it’s mostly mythical. There really isn’t much here, just some initial research, some of which seems “interesting.”
  • One last thing. If the goal is to make pot legal, because there’s a group that wants to smoke pot for recreational purposes, that’s fine. But to do it on the back of trying to prove that cannabis has some medical value that isn’t based on real science? That’s just going to make people dispute all of your claims.

Editor’s note: This article was originally published in August 2012. It has been completely revised and updated to include more comprehensive information, to improve readability and to add current research.

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Michael Simpson

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