Cannabis and Breast Cancer: What the Research Says (2026)

By James Buddsey · Updated October 22, 2026

Cannabis does not treat or cure breast cancer: every study showing that THC or CBD slows breast tumors was done in cell cultures or mice, and no human trial has tested cannabis as a breast cancer treatment. Where cannabinoids do have human evidence is symptom relief, mainly chemotherapy nausea that standard drugs have not controlled. Anyone in treatment should talk to their oncologist before using cannabis or CBD because of possible drug interactions.

Breast cancer patients are among the most frequent users of cannabis during treatment, and much of the online material about it rests on a handful of lab studies from the 2000s. This page separates what those studies found from what has been shown in people, as of October 2026. It is part of our overview of cannabis and cancer research.

Evidence at a Glance

What Lab Studies Found About Cannabis and Breast Cancer

Breast cancer is one of the most studied cancers in cannabinoid research, but all of the anticancer findings so far come from the lab.

• CBD in breast cancer cells (2006). An Italian team tested five plant cannabinoids on cancer cell lines and found CBD was the most potent at slowing growth, with less effect on normal cells. In mice carrying human breast tumors, CBD and a CBD-rich extract slowed tumor growth and reduced spread to the lungs.
• CBD and the Id-1 gene (2007). Researchers in San Francisco reported that CBD turned down Id-1, a gene linked to how aggressively breast cancer cells invade, in cell culture.
• THC in HER2-positive mice (2010). A Spanish group found that THC and a CB2 receptor drug reduced tumor growth and lung metastases in a mouse model of HER2-positive breast cancer. They also found the CB2 receptor in 91% of the human HER2-positive tumors they examined.

NCI’s PDQ summary notes that CBD killed both estrogen receptor-positive and -negative breast cancer cells while sparing normal breast cells in lab work. These are legitimate findings that justify further research. They are not evidence that cannabis treats breast cancer in a person.

Why lab results do not translate directly

Cell studies use concentrations that may not be reachable by smoking, vaping or eating cannabis, and mouse tumors are not human tumors. NCI also notes that in mice with a working immune system, THC sometimes suppressed the immune response against tumors and made them grow faster. Until a controlled human trial is done, the effect on real breast cancer is unknown.

Is There Any Human Evidence That Cannabis Treats Breast Cancer?

No. There are no published randomized trials testing cannabis, THC or CBD as a treatment for breast cancer. The National Academies found insufficient evidence that cannabinoids treat any cancer, and the 2024 ASCO guideline tells clinicians to recommend against cannabis as a cancer-directed treatment outside a clinical trial. Anecdotal stories of tumors shrinking on cannabis oil usually involve people who also had standard treatment, and they cannot show cause and effect.

Where Cannabis Can Help: Treatment Side Effects

Nausea and vomiting

This is the best-supported use. The FDA has approved dronabinol and nabilone for chemotherapy nausea and vomiting in patients who have not responded to standard antiemetics. A 2020 Australian randomized trial, in which 78% of participants were women, found that adding an oral THC:CBD extract to standard antiemetics increased the complete response rate from 14% to 25%. Many participants preferred it to placebo even though about a third reported moderate or severe sedation, dizziness or disorientation. ASCO supports cannabinoids for refractory nausea only as an add-on to guideline-based antiemetics.

Pain, sleep and anxiety

Many breast cancer patients report using cannabis for pain, sleep and anxiety. Controlled trials have not shown a clear benefit for cancer pain: a 2023 Cochrane review found that nabiximols and THC did not reduce opioid-resistant cancer pain more than placebo. Evidence for sleep and anxiety in cancer is limited to small studies. ASCO calls these uses uncertain.

Appetite

Trials of THC for cancer-related appetite loss have been mostly negative. In one large trial, megestrol outperformed dronabinol for both appetite and weight.

Cannabis and Breast Cancer Medications

Breast cancer treatment often involves long courses of oral drugs, including tamoxifen, aromatase inhibitors, CDK4/6 inhibitors and other targeted therapies. CBD inhibits several cytochrome P450 liver enzymes that process many medicines, and NCI warns that concentrated CBD oils could raise the toxicity or lower the effect of drugs metabolized this way. Tamoxifen, for example, has to be converted by liver enzymes into its active form. Whether everyday CBD doses change that in a clinically meaningful way has not been well studied, which is exactly why your oncologist and pharmacist should know what you take.

Observational studies have also linked cannabis use with a lower response rate to the immunotherapy drug nivolumab. Immunotherapy is used in some triple-negative breast cancers, so flag cannabis use before starting it.

Does Cannabis Raise Breast Cancer Risk?

There is no good evidence either way. The National Academies’ 2017 review reached no conclusion on cannabis and breast cancer, and the larger studies of cannabis and cancer risk, summarized in a 2019 JAMA Network Open meta-analysis, focused mainly on lung, head and neck and testicular cancers. Cannabis is not a known breast cancer risk factor, but that reflects a lack of research rather than proof of safety.

Risks and Side Effects

• Delaying or replacing treatment is the most serious risk. Breast cancer is highly treatable when caught early.
• Drug interactions with hormone therapy, targeted therapy and chemotherapy, especially with high-dose CBD.
• Drowsiness, dizziness and confusion, which add to fatigue from treatment and raise fall risk.
• Anxiety or paranoia with high-THC products, plus the risk of cannabis use disorder with heavy use, per NIDA.
• Smoke exposure, which the CDC says carries many of the same toxins as tobacco smoke. Oral products or tinctures avoid this; see our guide to tinctures.

Talk to Your Doctor

ASCO’s guideline encourages oncologists to talk about cannabis openly. Tell your team the product, the THC and CBD content, and how often you use it. Ask whether a prescription cannabinoid makes more sense for nausea, and whether any of your medications interact with CBD. If you want to explore cannabinoids as a cancer treatment, the right route is a clinical trial; NCI keeps a searchable database. Our piece on CBD benefits and hype covers CBD more broadly.

Breast Cancer and Cannabis FAQ

For adults 21+ in legal states. General information, not medical advice. Talk to a doctor before using cannabis for a health condition.