Cannabis has not been shown to shrink tumors in people: the tumor-shrinking results come from cell cultures and animals, and the few human trials, mostly in brain tumors, were too small to prove any benefit. The most promising human result is a 2021 phase 1b trial in recurrent glioblastoma that hinted at longer survival but was not designed to prove it. No cannabis product is approved to treat any tumor.
“Cannabis shrinks tumors” is one of the most repeated claims in cannabis culture. It started with real experiments: a 1975 study of lung tumors in mice and a 2000 study of brain tumors in rats. This page traces what those studies and the ones that followed actually showed, with a focus on gliomas, the brain tumors where the most human research has been done. It reflects the evidence as of October 2026 and is part of our overview of cannabis and cancer.
Evidence at a Glance
| Claim | Evidence level | Source |
|---|---|---|
| THC shrinks gliomas in rats and mice | Preclinical only | Galve-Roperh 2000 |
| THC plus CBD works better than either alone on glioblastoma cells | Preclinical only (cell lines) | Marcu 2010 |
| THC injected into recurrent glioblastoma is safe | Very limited (9-patient pilot, no control group) | Guzmán 2006 |
| Nabiximols with temozolomide improves glioblastoma survival | Limited (21-patient randomized phase 1b, not powered for survival) | Twelves 2021 |
| Cannabinoids treat any cancer in people | Insufficient evidence | NASEM 2017, ASCO 2024 |
Where the “Cannabis Shrinks Tumors” Claim Comes From
The 1975 mouse study
Researchers publishing in the Journal of the National Cancer Institute found that oral THC, delta-8-THC and cannabinol slowed the growth of lung tumors in mice and extended survival by 25% to 36%. CBD had no effect on tumor growth in that study, and THC did not extend the lives of mice with one type of leukemia. Activists later described this research as suppressed. In reality it was published in a leading journal and the field continued, though slowly, partly because cannabis research faced heavy federal restrictions.
The 2000 rat glioma study
A team at Complutense University in Madrid, led by Manuel Guzmán, reported in Nature Medicine that THC and a synthetic cannabinoid injected into the tumor caused considerable regression of malignant gliomas in rats and mice, without obvious brain toxicity. The mechanism involved cannabinoid receptors and a buildup of ceramide that pushed tumor cells toward self-destruction. This is the study behind many “THC destroys brain tumors” headlines.
Later lab work
Researchers in San Francisco found that combining CBD with THC slowed glioblastoma cells more than THC alone. NCI’s PDQ summary cites a meta-analysis of 34 glioma studies in cells and animals in which all but one found that cannabinoids selectively killed tumor cells. A 2012 Nature Reviews Cancer review describes similar effects across breast, lung, pancreatic and other tumor models.
Why Lab Results Are Not Enough
- Delivery. The rat studies injected cannabinoids directly into tumors. Smoking or eating cannabis delivers far lower, variable amounts to the brain.
- Immune effects. NCI notes that in mice with intact immune systems, THC sometimes suppressed antitumor immunity and made tumors grow faster.
- Translation. Most cancer drugs that work in animals fail in human trials. Animal results tell researchers what to test, not what works.
The Human Brain Tumor Trials
Spain, 2006: THC injected into recurrent glioblastoma
Guzmán’s team ran the first human trial of a cannabinoid as a tumor treatment. Nine patients whose glioblastoma had returned after surgery and radiation had THC delivered directly into the tumor through a catheter. The treatment was safe and did not cause obvious psychoactive effects. Median survival from the start of THC was 24 weeks. There was no control group, so the trial could not show whether THC helped.
UK, 2021: nabiximols with temozolomide
A UK team tested nabiximols, a mouth spray with roughly equal THC and CBD, alongside the standard chemotherapy drug temozolomide in recurrent glioblastoma. In the randomized part of this phase 1b trial, 12 patients got nabiximols and 9 got placebo. Six-month progression-free survival was 33% in both groups, but one-year survival was 83% with nabiximols versus 44% with placebo. Side effects included vomiting, dizziness and fatigue. The authors said the difference supports a larger, properly powered trial. With 21 patients, chance alone could explain it.
Other human data
NCI’s summary also describes a small 2016 case series of nine brain tumor patients taking CBD alongside standard treatment, and a phase II study of 88 glioma patients comparing two THC:CBD ratios for quality of life, which found no difference in the main outcome. Neither tested whether cannabis shrinks tumors.
What Major Bodies Conclude
The National Academies found insufficient evidence to say whether cannabinoids are an effective treatment for cancers, including glioma. The American Society of Clinical Oncology’s 2024 guideline tells clinicians to recommend against using cannabis or cannabinoids as a cancer-directed treatment unless it is within a clinical trial. The FDA has not approved any cannabinoid to treat a tumor, and NCI notes that concentrated cannabis oils marketed as cancer cures have never been tested for anticancer effect or safety.
What Cannabis Can Do for People With Tumors
The proven role is symptom relief. The FDA approved dronabinol and nabilone for chemotherapy nausea and vomiting that does not respond to standard drugs, and ASCO supports cannabinoids as an add-on for refractory nausea. Evidence for pain, appetite and sleep is weaker; a 2023 Cochrane review found that nabiximols and THC did not beat placebo for opioid-resistant cancer pain. Some brain tumor patients take anti-seizure drugs, and prescription CBD (Epidiolex) is FDA-approved only for specific childhood epilepsy syndromes, not tumor-related seizures.
Risks and Side Effects
- Skipping proven treatment in favor of cannabis oil is the most serious risk.
- Drug interactions. CBD inhibits liver enzymes that process many drugs, including some anti-seizure medicines and chemotherapy. NCI warns that concentrated CBD oils could change drug toxicity or effect.
- Brain-related side effects. THC can cause confusion, memory problems, dizziness and anxiety, which may be harder to tell apart from tumor symptoms. NIDA also notes the risk of psychosis with high-THC products in some people.
- Immunotherapy. Observational studies have linked cannabis to lower response rates to nivolumab.
- Cost and quality. Unregulated “cancer oils” can be expensive and their actual contents vary.
Talk to Your Doctor
If you or a family member has a brain or other tumor and wants to try cannabinoids, start with the oncology team. Ask whether a clinical trial is open; NCI’s trial search lists current studies. Ask how CBD or THC could interact with temozolomide, anti-seizure drugs or steroids, and whether a prescription cannabinoid makes sense for nausea. For more on the compounds themselves, see our explainers on THC and other cannabinoids and CBD benefits and hype, or browse the cannabis research hub.
Cannabis and Tumors FAQ
Does THC shrink tumors?
In rats, mice and cell cultures, yes, in several tumor types. In people, no study has shown that THC shrinks tumors; the human trials so far were small and not designed to prove it.
Can cannabis treat glioblastoma?
Not proven. A 2021 phase 1b trial found higher one-year survival with nabiximols plus temozolomide (83% vs. 44%), but with only 21 randomized patients it needs a larger trial to confirm.
Was cannabis cancer research suppressed in 1974?
The early mouse study was published in 1975 in the Journal of the National Cancer Institute. There is no evidence results were hidden; cannabis research in the US has been slowed in part by federal restrictions on the plant.
Does CBD oil shrink tumors?
There is no human evidence that it does. NCI notes that concentrated cannabis oils sold as cancer cures have not been tested in clinical trials.
Is there an FDA-approved cannabis drug for tumors?
No. FDA-approved cannabinoids are for chemotherapy nausea (dronabinol, nabilone), AIDS-related appetite loss (dronabinol) and certain seizure disorders (Epidiolex).
For adults 21+ in legal states. General information, not medical advice. Talk to a doctor before using cannabis for a health condition.