A new laboratory study has found that cannabidiol, better known as CBD, dramatically impaired the ability of chemotherapy-resistant head and neck cancer cells to survive, spread and form new colonies. Researchers reported a 98 percent reduction in colony formation after exposing cisplatin-resistant cancer cells to CBD, while additional experiments showed reduced migration, invasion and three-dimensional tumor growth. CBD also appeared to work synergistically with cisplatin, raising the possibility that the non-intoxicating cannabis compound could someday be investigated as an adjunct to conventional chemotherapy.
The findings, published October 26, 2026 in Medical Oncology by Edilene S. A. de Melo and colleagues at Brazil’s Federal University of São João del Rei, are entirely preclinical. The researchers studied human cancer cells in laboratory cultures, three-dimensional tumor spheroids and a chick-embryo membrane model—not patients with cancer. The headline 98 percent figure refers specifically to the cancer cells’ ability to form colonies after treatment, rather than a 98 percent reduction in tumors inside humans. Even with that important limitation, the results add to a growing body of research suggesting CBD can interfere with biological processes that help some chemotherapy-resistant cancers survive.
Researchers Targeted a Particularly Difficult Form of Head and Neck Cancer
The study focused on head and neck squamous cell carcinoma, or HNSCC, a broad group of cancers affecting areas including the mouth, throat, larynx, nasal cavity and related tissues. These tumors can be biologically very different depending on whether they are associated with human papillomavirus. HPV-positive head and neck cancers often respond better to treatment, while HPV-negative tumors are frequently associated with tobacco and alcohol exposure and tend to have a poorer prognosis.
Researchers used the HN13 cell line, an HPV-negative model with resistance to cisplatin, one of the most widely used chemotherapy drugs for head and neck cancer. Treatment resistance is a major clinical problem because a tumor can initially respond to chemotherapy before surviving cells evolve or select for mechanisms that make the drug much less effective. Once resistance develops, recurrence and metastatic disease become significantly more difficult to control.
Cisplatin kills cancer cells largely by damaging DNA, but malignant cells can resist it through enhanced DNA repair, altered drug transport, reduced cell-death signaling and other adaptations. Finding ways to resensitize those cells—or attack them through an entirely different biological pathway—has therefore become an important area of cancer research.
CBD Was More Toxic to Cancer Cells Than to Normal Cells
The researchers first exposed HN13 cancer cells to increasing concentrations of CBD for 24 hours. Cancer-cell viability declined as CBD concentration increased, producing an IC50 of approximately 10 micromolar. The IC50 is the concentration needed to reduce cell viability by about half under the experimental conditions.
Normal human fibroblasts and keratinocytes required substantially higher CBD concentrations to reach the same level of toxicity. The IC50 was approximately 27.8 micromolar in WI38 fibroblasts and 29.5 micromolar in HaCaT keratinocytes. That gave CBD selectivity indices of approximately 2.7 and 2.8, indicating that the resistant cancer cells were considerably more sensitive to CBD than the noncancerous cells used for comparison.
Cisplatin showed a different pattern. HN13 cells were highly resistant to the chemotherapy drug, requiring concentrations around 65 to 76 micromolar to reduce viability by half in different parts of the experiment. Normal keratinocytes were substantially more sensitive to cisplatin than the resistant cancer cells. That lack of selectivity illustrates one of chemotherapy’s persistent challenges: treatments powerful enough to damage malignant cells can also injure healthy rapidly dividing tissues.
CBD Reduced Cancer Cell Colony Formation by 98 Percent
The study’s most dramatic result came from its clonogenic assay, which examines whether cancer cells can survive an initial treatment and later reproduce sufficiently to establish colonies. Researchers treated HN13 cells with 10 micromolar CBD for 24 hours, removed the treatment and then allowed the surviving cells to recover and grow for five days.
Colony formation fell by 98 percent compared with untreated controls. This is an important distinction from a basic cell-viability test. A treatment might temporarily slow cancer cells without permanently eliminating their ability to proliferate. A colony-forming assay instead asks whether cells retain the long-term reproductive capacity necessary to rebuild a population after treatment.
The result suggests CBD caused lasting damage to the proliferative potential of many of the chemotherapy-resistant cells under laboratory conditions. It does not mean CBD eliminated 98 percent of a human tumor or that taking CBD oil would reproduce the same response in a patient. The experimental concentration, direct exposure and controlled environment are very different from the pharmacology of oral CBD in the human body.
CBD Also Suppressed Migration and Invasion
Cancer lethality depends on more than uncontrolled cell division. Tumor cells become particularly dangerous when they gain the ability to invade surrounding tissues and migrate to distant organs. The researchers therefore examined whether CBD affected behaviors associated with metastasis.
Treatment with 10 micromolar CBD significantly reduced HN13 invasion through laboratory membranes and substantially slowed migration in wound-healing experiments. After 72 hours, untreated cells had almost completely moved across the artificial wound created in the culture, while CBD-treated cells still left approximately 66 percent of the original gap open.
CBD also reduced the activity of matrix metalloproteinases MMP-2 and MMP-9. MMPs are enzymes capable of breaking down components of the extracellular matrix surrounding tissues, helping malignant cells invade neighboring structures. Pro-MMP-9 activity fell approximately 57 percent, active MMP-9 declined roughly 77 percent and MMP-2 activity dropped approximately 45 percent. These findings suggest CBD was affecting several processes related to cancer aggressiveness rather than simply slowing cell division.
Three-Dimensional Tumor Models Also Responded to CBD
Ordinary cancer-cell experiments are typically conducted in flat dishes where cells grow in a two-dimensional layer. Although useful, that environment does not accurately reproduce the physical structure of a tumor. Researchers therefore tested CBD on three-dimensional HN13 spheroids, which allow cancer cells to interact with one another in a more tumor-like arrangement.
Untreated spheroids continued expanding over 72 hours, while CBD significantly suppressed their growth. Three-dimensional models are usually harder to treat than isolated cells because the compound must penetrate multiple layers and because interactions between neighboring cells can enhance survival. Seeing activity in spheroids therefore provides a stronger preclinical signal than a simple monolayer experiment alone.
The researchers took the work another step using the chorioallantoic membrane, or CAM, model. This system grows tumor cells on the highly vascular membrane of a developing chick embryo and is commonly used to evaluate tumor growth and blood-vessel formation. CBD reduced both tumor growth and vascular area in the model without producing signs of vascular irritation in a separate safety assay. Those results suggest the compound may also interfere with the blood-vessel support tumors require as they expand.
CBD and Cisplatin Produced Synergistic Effects
Perhaps the most clinically interesting aspect of the study involved combining CBD with the chemotherapy drug the HN13 cells had become resistant to. Researchers tested several CBD and cisplatin combinations and calculated a combination index to determine whether the two compounds simply added their effects together or produced true pharmacological synergy.
The strongest synergy occurred when 10 micromolar CBD was combined with 5 micromolar cisplatin, producing a combination index of 0.6. Another combination using 10 micromolar CBD with 25 micromolar cisplatin produced an index of 0.8. Values below 1 indicate synergy, meaning the combined effect was stronger than expected from the two treatments acting independently.
If a similar interaction could eventually be demonstrated in animals and humans, one potential advantage would be restoring the usefulness of chemotherapy against resistant tumors or achieving comparable anticancer activity with less cisplatin. That could be important because cisplatin can cause kidney injury, hearing loss, nerve damage, nausea and other serious adverse effects. The current study does not establish that CBD allows doctors to lower chemotherapy doses in patients, but it provides a reason to investigate that possibility.
CBD Altered Apoptosis, Autophagy and Cell-Cycle Signaling
To understand how CBD was producing these effects, the researchers examined proteins involved in programmed cell death, cellular recycling and proliferation. CBD-treated HN13 cells displayed physical signs associated with cell death, including shrinkage, chromatin condensation, membrane blebbing and nuclear fragmentation. Only about 6 percent of the measured cells remained in the viable category after treatment, while approximately 83 percent displayed characteristics associated with early-stage cell death.
CBD also altered several proteins involved in apoptosis, the tightly controlled process through which damaged or abnormal cells dismantle themselves. Researchers detected increased cleavage of caspase-3 and changes involving PARP, both commonly used markers of apoptotic signaling. Cyclin D1, a protein that helps drive cells through the division cycle, fell approximately 76 percent.
The results involving autophagy were more complicated. CBD strongly increased LC3 and Beclin-related signaling associated with the cellular recycling process known as autophagy. When researchers inhibited that pathway with 3-methyladenine, CBD produced even stronger apoptotic signaling and lower cell viability. That suggests autophagy may partly function as a defensive response allowing some cancer cells to survive CBD-induced stress rather than serving solely as a mechanism of death.
Earlier Head and Neck Cancer Research Pointed in the Same Direction
The 2026 findings build on an important 2020 study by Yoon Young Go and colleagues published in Scientific Reports. That research tested CBD in several human head and neck squamous cell carcinoma lines and found dose- and time-dependent reductions in cell viability, migration and invasion.
Go and colleagues also tested CBD in mouse xenograft models. Animals carrying head and neck tumors showed reduced tumor growth after CBD treatment, while the combination of CBD and cisplatin produced stronger tumor suppression than either approach alone. The researchers also reported synergy when CBD was paired with cisplatin, 5-fluorouracil or paclitaxel in laboratory cells.
The earlier study involved chemotherapy-sensitive models rather than focusing specifically on cisplatin-resistant HN13 cells. The new Medical Oncology research therefore extends the evidence into a more difficult treatment setting and provides additional mechanistic information involving invasion, metalloproteinases, clonogenic survival and autophagy.
Similar Findings Have Appeared in Other Chemotherapy-Resistant Cancers
CBD’s possible effects on drug-resistant cancer are not limited to head and neck tumors. A 2022 study by Swati Misri and colleagues at Ohio State University examined cisplatin-resistant non-small cell lung cancer. Published in Cancers, the research found that CBD promoted apoptosis and reduced tumor progression and metastasis in a mouse xenograft model. The scientists implicated the TRPV2 receptor and changes in NRF2 and reactive oxygen species as important components of the response.
In 2024, researchers studying gemcitabine-resistant cholangiocarcinoma, a cancer of the bile ducts, found that CBD suppressed colony formation and tumor growth while increasing cellular stress pathways associated with apoptosis. The work showed reduced tumor growth in xenografted mice at CBD doses of 10 and 40 milligrams per kilogram and linked the effect partly to endoplasmic-reticulum stress.
Research involving drug-resistant breast cancer has produced additional signals. Studies of doxorubicin-resistant triple-negative breast cancer have found that CBD, alone or alongside other cannabinoids such as THCV or CBC, can increase chemotherapy sensitivity and inhibit signaling pathways associated with survival and metastasis. A 2026 laboratory study of doxorubicin-sensitive and resistant breast cancer cells likewise found CBD reduced colony formation, invasion and proliferation while altering genes involved in apoptosis, DNA repair, metabolism and cell-cycle control.
Not Every CBD-Chemotherapy Combination Has Been Beneficial
The growing body of anticancer research also contains an important warning: CBD does not universally strengthen chemotherapy. A 2023 preclinical study examining platinum drugs found that CBD and cannabis extracts could actually reduce the effectiveness of cisplatin, oxaliplatin and carboplatin in certain cell models. Researchers linked the effect to reduced intracellular accumulation of platinum compounds.
A 2025 study of platinum-resistant ovarian cancer similarly found that CBD could produce either synergy or antagonism depending on the cancer cell line, drug concentration and timing. Simultaneously administering CBD with cisplatin was antagonistic in some models, while sequential treatment showed more favorable results.
These conflicting findings demonstrate why cannabinoids cannot simply be assumed to improve chemotherapy. Cancer biology varies enormously between tumor types, and drug interactions can change according to concentration, formulation and treatment schedule. A combination that is strongly synergistic in one cancer cell may be neutral or even counterproductive in another.
The CBD Concentration Is a Major Translational Limitation
The authors of the new study specifically acknowledged one of its most important limitations. The primary experimental CBD concentration was 10 micromolar, equivalent to approximately 3,145 nanograms per milliliter. That is higher than plasma CBD concentrations normally achieved in humans.
Laboratory cells are directly immersed in a known CBD concentration, while CBD taken orally must be absorbed through the digestive system, metabolized in the liver, circulate through the blood and ultimately reach the tumor. Only a fraction of an oral dose may arrive at the cancer at the concentration used in a cell dish. This is one of the most common reasons promising laboratory anticancer compounds fail when tested in people.
A 2024 study examining CBD across human cancer cells and patient-derived tumor slices reached a similarly cautious conclusion. Researchers found that high CBD concentrations could promote apoptosis and suppress oncogenic signaling but reported minimal effects at concentrations considered more physiologically achievable. Determining whether therapeutic concentrations can safely reach tumors therefore remains a crucial unanswered question.
CBD Is Not an Established Cancer Treatment
Cannabis and cannabinoids are already used in oncology, but primarily for symptom management rather than to destroy tumors. THC-based medicines can help some patients with chemotherapy-induced nausea and vomiting, while cannabis products are commonly used for pain, appetite problems, sleep disturbance and other cancer-related symptoms.
The National Cancer Institute continues to state that laboratory and animal studies have identified potentially important anticancer properties of cannabinoids, but there is not sufficient clinical evidence to conclude that cannabis or CBD treats cancer in humans. No CBD product is currently approved as an anticancer therapy.
That distinction is especially important with research involving chemotherapy resistance. Patients should not substitute commercially available CBD products for surgery, radiation, chemotherapy, immunotherapy or other established cancer treatments based on laboratory findings. CBD can also interact with enzymes involved in metabolizing prescription drugs, making unsupervised use during cancer treatment potentially consequential.
The Next Step Is Stronger Preclinical Testing
The researchers behind the new study argue that the findings justify further preclinical investigation rather than immediate clinical use. Mammalian models will be particularly important because they can show whether CBD reaches resistant head and neck tumors at effective concentrations, whether the cisplatin synergy persists in living organisms and whether the combination produces unexpected toxicity.
Future research will also need to examine different CBD doses, treatment schedules and formulations. Determining whether CBD should be given before, during or after cisplatin could be critical, especially given evidence from other cancer models showing that timing can determine whether the interaction becomes synergistic or antagonistic.
If those experiments remain favorable, carefully designed early-stage human trials could eventually determine safety, pharmacokinetics and an appropriate dose before researchers attempt to establish whether CBD actually improves tumor response or survival.
A Striking Result That Still Has a Long Way to Go
The 98 percent reduction in colony formation is one of the more dramatic recent findings in preclinical cannabinoid cancer research, particularly because it occurred in cells already resistant to cisplatin. CBD also reduced migration, invasion, metalloproteinase activity and three-dimensional tumor growth while producing synergistic effects with cisplatin and altering multiple cell-death pathways.
Those results strengthen a broader pattern emerging from studies of head and neck, lung, breast, ovarian and bile-duct cancers: under certain experimental conditions, CBD can interfere with mechanisms that allow treatment-resistant cancer cells to survive.
But the gap between killing cancer cells in a laboratory and treating cancer in a patient remains enormous. The CBD concentrations used in the new experiments exceed typical human blood levels, and cannabinoids can interact with chemotherapy differently depending on the tumor and treatment schedule. For now, the study should be viewed as compelling evidence for further cancer research—not evidence that CBD oil can cure chemotherapy-resistant disease.
What researchers have demonstrated is more specific and scientifically valuable: a non-intoxicating cannabis compound can profoundly disrupt the long-term survival of a difficult, cisplatin-resistant head and neck cancer model and can make that model more vulnerable to chemotherapy. Whether scientists can turn that laboratory effect into a safe and effective cancer treatment is now the much harder question.
