Cannabis, CBD Oil and Cancer Risk: What Does the Research Say?

What does the research say?

It is not possible to test whether cannabis changes cancer risk by randomly assigning people to use cannabis and following them for years, so all of the evidence comes from observational studies. Case-control studies compare the past cannabis use of people diagnosed with cancer with that of people without cancer; cohort studies follow users and non-users for years; and Mendelian randomization uses genes that make people more likely to use cannabis as a kind of natural experiment. In these studies, seeing two things together does not show that one causes the other. The biggest difficulty is tobacco: most people who smoke cannabis also smoke cigarettes, in some regions cannabis is smoked mixed with tobacco, and few people smoke only cannabis. Use is mostly based on self-report, and newer registry studies measure not the amount used but a diagnosis of 'cannabis use disorder' in health records.

Laboratory comparisons found that cannabis smoke contains many substances that can cause cancer and that are also found in tobacco smoke, and that the physical and chemical properties of the two kinds of smoke are largely similar. On the other hand, THC itself did not cause cancer in a two-year rat study in which it was given by mouth; in mice there was only an 'equivocal' finding in the thyroid that was not dose-related. Taken together, these two findings suggest that the main concern for lung and head and neck cancers may be the products of burning; but this has not been tested directly in people. For the link seen with the testis, which smoke does not reach directly, researchers are discussing the role of both cannabinoids and the other substances in smoke.

Lung cancer: In an international analysis that combined data from six studies and included 2159 people with lung cancer, the risk did not increase in habitual cannabis smokers, and no increase was seen in people who had never smoked cigarettes either. The New Zealand study in the same pool, however, had on its own reported an increase with heavy use. In a Swedish study that followed around 49,000 men aged 18–20 for 40 years, the risk was more than twice as high in those who had smoked cannabis more than 50 times in their youth. In a 2026 US study based on health records, the risk was about four times higher in people with cannabis use disorder; but the amount of tobacco smoked is not known in these records, and letters questioning the method were published. In North Africa, where cannabis is smoked mixed with tobacco, the risk was high, but all cannabis smokers there also smoked cigarettes. A study using genetic predisposition showed a small increase in squamous cell lung cancer. A comprehensive 2019 review found the evidence for the lung insufficient, while the 2017 report of the US National Academies saw moderate evidence of no association. These two assessments were made before the newer registry studies that report an increase. A 2026 meta-analysis that reported an increase was not included in the table: two numbers that drive its result for cohorts do not match the source study's own results. The evidence is mixed and very weak.

Head and neck and oral cancers: In a large analysis that combined data from 4029 patients in five studies, smoking cannabis did not increase the risk of head and neck cancer, and there was no increase in people who had never used cigarettes or alcohol either. In another pool of nine studies, a slight increase was seen in the tonsil and base-of-tongue area of the throat (oropharynx), but this increase disappeared once HPV infection was taken into account; for tongue cancer, the risk was lower. Meta-analyses also found no overall increase, the 2017 report of the US National Academies also saw moderate evidence of no association, and a 2025 meta-analysis reported a lower risk of oral cancer. In contrast, in 2024 and 2025 US studies based on health records, any head and neck cancer was about 3.5 times and oral cancer 2.5–3.3 times more common in people with cannabis use disorder (higher for the larynx); these records do not show the amount of tobacco and alcohol used or HPV status. An increase seen in another hospital registry disappeared once alcohol or smoking was taken into account. In North Africa, the link with cancer of the upper throat behind the nose (nasopharynx) was clearer for cannabis smoked mixed with tobacco. The evidence is conflicting and very weak.

Testicular cancer: This is the type of cancer with the most consistent finding. In meta-analyses that combined the same three US case-control studies (so they do not count as independent evidence from one another), long-term (more than 10 years), frequent (at least weekly) or current use was linked to about a 1.4–1.9-fold increase in testicular germ cell tumors; the link was stronger for nonseminoma tumors. In Swedish men followed for 42 years, use more than 50 times in youth was linked to a higher risk, while ever having used was not. In a 2026 study from France, an increase was seen with daily or regular use in young adulthood, but this time the link appeared for seminoma rather than nonseminoma tumors; in a large UK cohort no link was found, but there participants were aged 40–69 and there were only 29 testicular cancers. The comprehensive 2019 review rated the strength of evidence as low, and the 2017 report of the US National Academies considered it 'limited' for the nonseminoma type. Because testicular cancer is rare, relative increases add up to a small absolute risk: in the Swedish study it occurred in 119 of 45,000 men over 42 years.

Bladder, prostate and kidney cancers: The findings point in two directions. In a California study that followed about 84,000 men for 11 years, bladder cancer was less common in those who used only cannabis; in a UK study that followed more than 150,000 people, prostate and kidney cancer was also less common in those who had used cannabis before. In contrast, in a 2025 US study based on health records, bladder, kidney and prostate cancer were more common in people with cannabis use disorder. In a 1997 US cohort, there was an increase in prostate cancer only among non-smokers. For the bladder, the only systematic review found few and conflicting studies. The evidence is insufficient.

Brain tumors: In a single cohort in California that followed more than 133,000 people for up to 21 years, the risk of glioma (a type of brain tumor) was about three times higher in those who smoked cannabis at least once a month, but the risk did not rise with more use; a study using genetic predisposition showed no causal link for glioma. The evidence is insufficient.

Use during pregnancy and childhood cancers: In five case-control studies carried out in the US between 1989 and 2006, the mother's use of marijuana or other recreational drugs around the time of pregnancy (in one, the father's as well) was linked to a type of leukemia, rhabdomyosarcoma (a soft tissue cancer) and neuroblastoma in the child; the increase seen for astrocytoma (a brain tumor) was not statistically significant. However, the largest study, done to test the first report on childhood leukemia, did not confirm the increase. These studies were small, use was reported from memory years later, and in some studies marijuana had been used together with other drugs; no new cohort study was found. The 2017 report of the US National Academies also assessed these studies and found the evidence insufficient. The evidence is insufficient. Cannabis use during pregnancy was, however, more consistently linked to other outcomes such as low birth weight; these estimates were not adjusted for confounders.

All cancers and findings in the opposite direction: In a 1997 cohort that followed a young population for an average of 8.6 years, there was no increase among cannabis users when all cancers were considered. A 2025 cancer-specific umbrella review wrote that the links between cannabis and cancer are 'suggestive rather than conclusive' and that the closest association was seen with nonseminoma testicular cancer; in another umbrella review from 2023 that assessed 101 meta-analyses, no cancer-related association reached the strong evidence class. Some studies reported that bladder, prostate, kidney, oral or pancreatic cancer was less common in cannabis users; a 2021 meta-analysis also found a lower overall risk when some studies were excluded. These opposite findings are also observational and should be read with care: in some, adjustment for confounders was limited, in one the follow-up of cannabis users was shorter, and the meta-analysis result depended on which studies were excluded. It has not been shown that cannabis or CBD prevents cancer. In most studies using genetic predisposition, there was either no link or a very small one.

Pure CBD: No study was found that looked at cancer risk from CBD oil or pure CBD in people. The US and European product information describe the two-year animal studies of the approved pure CBD medicine in different detail. According to the US label, there was no statistically significant increase in tumors in rats; benign liver tumors increased in male mice at the highest dose, and at one dose lower, the blood CBD level was close to that at the highest doses of the medicine used in people. According to the European product information, however, benign liver tumors increased in male mice at all doses and in female mice at the highest dose; in male rats, overgrowth and benign tumors of the Leydig cells in the testis increased at all doses, including the lowest dose, at which the blood CBD level was lower than in people, and a malignant tumor was seen in one rat at the highest dose. The European product information states that the relevance of the Leydig cell tumors in rats to humans is generally considered limited; neither document explains the reason for the difference between them. CBD was negative in genotoxicity (genetic damage) tests. In 2026, the European Food Safety Authority wrote that, despite publications suggesting that pure CBD itself does not cause genetic damage, manufacturing and formulation may introduce new hazards, and that the effects of long-term use are unknown. The effect of CBD on the course of an existing cancer is covered on the site's 'Cancer Treatment and CBD' page.

Strength of evidence: ○○○○ Insufficient evidence. Finding: Not studied enough. What does this mean?

Possible side effects and drug interactions

Because this page describes a risk rather than a treatment, 'side effect' here means the harms of cannabis use other than cancer. In an umbrella review that assessed more than one hundred meta-analyses, the strongest evidence was found for use during pregnancy and low birth weight and small-for-gestational-age babies, for crashes among people who drive, and for psychosis in the general population; in randomized trials of cannabis-based medicines, nervous system side effects such as dizziness and drowsiness, as well as psychiatric side effects, also increased. The effects of smoked cannabis on the heart and blood vessels and on mental health are covered separately on the related pages of the site.

For pure CBD, the European Food Safety Authority reported consistent liver effects in animal studies and, in human studies, a possibility of liver harm, especially when used together with other medicines. CBD can slow the breakdown of some medicines in the body; it is important for people who take medicines regularly, especially those being treated for cancer, to talk to their doctor before starting.

To check interactions with the medicines you take: CBD drug interaction checker.

How was CBD used in studies?

This page describes a risk, not a treatment, so there is no tested or recommended dose. The studies measured cannabis use in different ways: joint-years (use equal to smoking one joint a day for one year), the number of times used over a lifetime (e.g., more than 50 times), at least weekly or daily use, duration of use (e.g., more than 10 years), or a diagnosis of 'cannabis use disorder' in health records.

In studies that found an increase, the link was mostly seen with long-term, frequent or heavy use (for testicular cancer, use for more than 10 years or at least weekly; in the Swedish studies, more than 50 times over a lifetime for lung and testis); in most studies, ever having used was not linked to risk. Because there are no human studies of cancer risk with pure CBD, no amount can be called safe or risky. In animal studies, according to the US label, the increase in male mice was seen at the highest dose (300 mg per kilogram per day), and at one dose lower the blood CBD level was about 1–2 times that at the highest human doses of the approved medicine. According to the European product information, however, the increase was seen in male mice at all doses (at the highest dose, the blood level was about 3 times that at the human dose) and, in the testes of male rats, also at the lowest dose, at which the blood level was lower than at the human dose. These are animal study doses, not personal recommendations.

Bottom line

Whether pure CBD raises cancer risk is not known: there are no studies looking at this in people; in two-year animal studies, benign liver tumors increased in mice and, according to the European product information, benign tumors of the testis also increased in male rats, and what this means for people is not known. For smoked cannabis, the most consistent finding is a link with testicular cancer after long-term or frequent use; results for lung and head and neck cancers are mixed and hard to separate from the effect of tobacco; and for some cancers there are findings in the opposite direction. The strength of evidence is weak. Cannabis smoke contains many of the harmful substances in tobacco smoke, and the highest risk was seen in people who breathe in both kinds of smoke together; talking through your questions with your doctor is the safest path.

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