In laboratory studies, cannabis compounds kill cancer cells in a dish. They only do this at concentrations higher than human blood reaches. No adequate human trial shows that cannabis treats cancer. This part explains why. It also covers what happens to patients who replace treatment, which drug interactions matter, and how contaminated cannabis endangers people with weak immunity.
Part 1 showed that the evidence supports only one narrow role: adding cannabinoids to standard drugs when chemotherapy sickness is not controlled. For pain, appetite, sleep and anxiety, the evidence is negative or insufficient.
This is Part 2. Part 1: Cannabis and Cancer: What the Evidence Supports and What It Does Not
8. What Cannabinoids Do to Cancer Cells in the Laboratory
Ramer and Hinz reviewed this research in the British Journal of Cancer in 2022. The studies used cell lines, which are cancer cells grown in a laboratory. In these cells, cannabinoids (the active compounds of cannabis) did three things:
- Triggered apoptosis, the cell’s own programmed death. This happened through a build-up of a fat molecule called ceramide and through stress in the part of the cell that makes proteins (the endoplasmic reticulum).
- Slowed cell growth. They blocked a growth signal called Akt and stopped the cell’s division cycle.
- Blocked angiogenesis, the growth of new blood vessels that a tumour needs for its blood supply.
The concentration problem: the core of the argument
The same review points out a key gap. Most studies that showed cancer cells dying used concentrations above 1 micromolar, a unit of concentration in a liquid. After THC is inhaled or swallowed, the highest level in the blood does not go above 1 micromolar. See Cannabinoid Pharmacokinetics for how blood levels depend on the route.
Taking more cannabis does not close this gap. The body absorbs only so much, and side effects become intolerable before the level is high enough.
The review also records the opposite effect. Human papillomavirus (HPV) is a virus that causes some head and neck cancers. In laboratory models of these cancers, THC and other cannabinoids made the cancer grow rather than shrink. The review also cites earlier laboratory work in which concentrations below 1 micromolar, the range human blood actually reaches, made cancer cells grow faster.
Two more gaps between the dish and the patient
Serum and plastic. Serum is the protein-rich part of blood. It is normally added to the liquid that feeds cells in a dish. Many cannabinoid studies use little or no serum. Carkaci-Salli and colleagues at Penn State, a US university, tested what difference this makes in a 2024 study, published as “Cannabinoids as Potential Cancer Therapeutics: The Concentration Conundrum” (Cannabis and Cannabinoid Research 9(4)).
The researchers tested four cannabinoids at 10 micromolar on six cancer cell lines. Three of them worked better without serum: THC, CBD and a laboratory-designed compound called KM-233. Their IC50, the concentration needed to stop half of the cells, was lower without serum. Without serum, THC and CBD also stuck heavily to the plastic of the dish.
The authors note that 90 to 95% of cannabinoids in the body are bound to proteins in the blood. Only the small unbound part can act on cells. A dish without serum leaves the whole dose free. So a common laboratory habit makes cannabinoids look stronger than they can be in a patient.
Immunity. A popular argument says cannabis helps the immune system fight cancer. At least one line of evidence points the other way. A 2022 study in Signal Transduction and Targeted Therapy looked at T cells, the white blood cells that kill cancer cells. Cannabis weakened their anti-tumour response by blocking JAK/STAT, a chain of signals that switches T cells on. It did this through CNR2, the gene for the CB2 receptor, a cannabinoid receptor found mainly on immune cells. See The Endocannabinoid System.
This matters more now that immunotherapy is central to treating many cancers.
What you can do: When a post claims cannabis “kills cancer cells”, look for the concentration in the study. Compare it with 1 micromolar. If the study used more, a person using cannabis does not reach that level.
9. Human Trials on the Tumour Itself
The 2006 brain tumour pilot study
In 2006, Guzmán and colleagues published a pilot study in the British Journal of Cancer. It included nine patients with recurrent glioblastoma, an aggressive brain tumour that had come back after treatment. THC was fed through a thin tube (catheter) straight into the space left after surgery removed the tumour. The researchers used this route because the bloodstream cannot carry enough THC to the tumour.
Median survival from the start of THC treatment was 24 weeks. Median means half the patients lived for less time and half for more. The US National Cancer Institute’s summary: the study showed no significant clinical benefit. See also Cannabinoids and Glioma.
The 2021 trial: what it showed and what it did not
In 2021, Twelves and colleagues reported a phase 1b trial in recurrent glioblastoma. A phase 1b trial is an early trial whose main aim is to check safety. Patients received the standard chemotherapy drug temozolomide plus either nabiximols spray or placebo. In part 2 of the trial, 21 patients were assigned by chance: 12 to nabiximols and 9 to placebo.
The figure shared online is one-year survival: 83% with nabiximols against 44% with placebo. The context is rarely shared with it:
- Only 21 people took part. The trial was too small to detect a real difference in survival.
- Two placebo patients died within 40 days. With only 9 people in that group, each death moves the result by about 11 percentage points.
- The share of patients alive without tumour growth at six months was the same in both groups: 33%.
The authors concluded only that the result justifies a trial large enough to give a real answer. An editorial in the same journal warned against careless off-label use, meaning prescribing a medicine for a purpose its licence does not cover.
That larger trial, called ARISTOCRAT, is under way in the United Kingdom. It compares temozolomide with and without cannabinoids in recurrent glioblastoma. Its protocol was published in BMC Cancer in 2024.
Translator’s note: All ARISTOCRAT trial sites are in the United Kingdom, and its registry entry shows it is no longer recruiting. A patient who wants a cannabinoid as part of cancer treatment can ask the oncologist about registered clinical trials. In a trial, the cannabinoid is added to standard treatment and the patient is monitored.
10. High-Dose Cannabis Oil Protocols
Many people searching this subject are looking for concentrated whole-plant cannabis oil, taken in rising amounts over a fixed period. A Canadian man made it popular in the late 2000s, and it carries his name. See Rick Simpson Oil. This article gives no amounts, schedule or method, so that a frightened person has nothing to follow instead of seeing an oncologist.
No randomised controlled trial of this protocol has been published. No controlled case series has been published either. Its claims that set amounts over a set number of days give set results are widely repeated but have no source.
Cancer Research UK, a British cancer research charity, states that the research so far has been laboratory work with mixed results. It says this work does not show whether cannabinoids can treat cancer in people. It adds that the quality of these preparations varies so much that nobody can know what one contains.
People turn to these protocols after being told their chances are poor. That is a human reaction, not a foolish one.
Translator’s note: Growers are sometimes asked to make “cancer oil” for a patient. Selling or processing a controlled herb without a licence breaks Section 46 of the Thai Traditional Medicine Protection Act B.E. 2542 (1999). Under Section 78, the penalty is up to 1 year in prison, a fine of up to 20,000 baht, or both. Separately, extracts with more than 0.2% THC can only be licensed to certain bodies. These are Thai juristic persons that are not “foreign” under the Foreign Business Act B.E. 2542, state agencies and the Thai Red Cross Society. This rule comes from the Ministerial Regulation on Cannabis or Hemp Extracts B.E. 2569, in force from 26 April 2026. If someone asks you for such an oil, decline and point them to their oncologist.
11. What Happens When Patients Replace Treatment
Johnson and colleagues published a study in the Journal of the National Cancer Institute in 2018. Using the US National Cancer Database, they found 281 patients with breast, prostate, lung or colorectal cancer that had not spread to distant organs. These patients chose alternative medicine as their only treatment. The study counted alternative medicine in general, not cannabis alone. They were matched with 560 similar patients who had conventional treatment.
The hazard ratio compares the risk of death between the two groups at any point in time. A hazard ratio of 2.50 means the risk of death was 2.5 times higher.
| Cancer type | Alive after five years, alternative medicine only | Alive after five years, conventional treatment | Hazard ratio for death |
|---|---|---|---|
| All four cancers | 54.7% | 78.3% | 2.50 |
| Breast | 58.1% | 86.6% | 5.68 |
| Colorectal | 32.7% | 79.4% | 4.57 |
| Lung | 19.9% | 41.3% | 2.17 |
| Prostate | — | — | 1.68 (not statistically significant, so it may be chance) |
The same group published a second analysis in JAMA Oncology in 2018. It looked at people who used complementary medicine alongside conventional treatment. Their extra risk came from refusing parts of conventional treatment, not from the complementary therapy itself. The danger is replacing treatment and delaying it. See When a Patient Declines Cancer Treatment in Favour of Cannabis.
What you can do: If someone you know plans to use cannabis instead of treatment, show them this table. Encourage them to bring it to their oncologist before they decide.
12. Drug Interactions That Matter in Cancer Care
Cytochrome P450 enzymes are liver enzymes that break down many drugs. THC is broken down mainly by the enzymes CYP2C9 and CYP3A4. CBD is broken down mainly by CYP2C19, with help from CYP2C9 and CYP3A4.
Both compounds also slow down several of these enzymes in laboratory tests. A 2023 review in Deutsches Ärzteblatt International, a German medical journal, lists them. THC slows 1A2, 3A4, 2B6, 2C9 and 2D6. CBD slows 3A4, 2C19 and 2C9. Many chemotherapy drugs, anti-sickness drugs and opioid painkillers use the same enzymes. If an enzyme slows down, another drug can build up in the blood. See Cannabis Drug Interactions.
| Drug | Reported effect | Why it matters |
|---|---|---|
| Warfarin (a blood thinner) | A 2022 review in the Journal of Clinical Medicine rated this the highest-risk documented cannabis interaction. INR rose sharply. INR is a blood test of how long blood takes to clot. | Risk of bleeding in people on blood thinners |
| Tacrolimus (a drug that suppresses the immune system) | CBD raised its blood level so much that the dose had to be cut sharply | Used after transplants and in graft-versus-host disease, where donor immune cells attack the patient’s body after a stem-cell transplant |
Cannabis also adds to the drowsiness caused by opioids and benzodiazepines (sleeping and anti-anxiety drugs). It can cause dizziness, which is serious in two groups: people with low platelets (thrombocytopenia), who bleed easily, and people whose cancer has spread to the bone, where a fall can cause a fracture.
Cannabis and immune checkpoint inhibitors
Immune checkpoint inhibitors are cancer drugs that release a brake on the immune system. This lets T cells attack the tumour. Nivolumab is one of them. The laboratory finding in section 8, that cannabis weakens T cells, raises the question of whether it could blunt these drugs. Two observational studies point that way.
Taha and colleagues (The Oncologist, 2019) compared 89 patients on nivolumab alone with 51 on nivolumab plus cannabis. The response rate was 37.5% on nivolumab alone and 15.9% with cannabis. Cannabis was the only significant factor linked to a lower response.
Bar-Sela and colleagues (Cancers, 2020) followed 102 patients starting immunotherapy, of whom 34 used cannabis. The median time until the cancer grew again was 3.4 months for cannabis users and 13.1 months for non-users. Median overall survival was 6.4 months against 28.5 months.
Both studies are observational and confounded: other differences between the groups could explain the results. For example, sicker patients may be more likely to use cannabis. Neither study proves harm. Still, two datasets point the same way. MASCC (the Multinational Association of Supportive Care in Cancer) advises caution with checkpoint inhibitors.
What you can do: Anyone on immunotherapy should tell the oncologist before using any cannabis product. The doctor can then weigh this signal against the patient’s own situation.
13. Contamination Risk for People With Weak Immunity
In a 2020 study in Emerging Infectious Diseases, Benedict and colleagues compared 53,217 people with a cannabis-use diagnosis code in their records against more than 21 million people without one. Fungal infections were recorded in 0.08% of users and 0.03% of non-users. After adjusting for age and weakened immunity, the odds ratio was 3.5. This means the odds of a fungal infection were about 3.5 times higher in users. The infections included aspergillosis and mucormycosis, serious fungal infections that can invade the lungs and other organs. The study design cannot prove cannabis caused them. The authors still warned of possible serious illness and death in people with weak immunity.
A 2023 review in Frontiers in Microbiology explains why. Over a hundred fungal species have been found on cannabis, including Aspergillus, Penicillium and Mucor. Some can produce mycotoxins (poisons made by moulds) such as aflatoxins and ochratoxin A. Heat from smoking or vaping may not kill all fungal spores.
Neutropenia is a very low level of neutrophils, the white blood cells that fight infection. Chemotherapy often causes it. In a neutropenic patient, invasive aspergillosis has a very high death rate. Pesticide residues, heavy metals and solvents also become concentrated in an oil along with the cannabinoids.
Translator’s note: Thailand’s rainy season brings high humidity, which raises mould risk during drying and storage. See Mycotoxins on Cannabis Flower. If you grow for patients, have each batch tested for mould, mycotoxins, pesticide residues and heavy metals. The test shows whether the batch is safe for a person with weak immunity. Learn to read the result in How to Read a Cannabis Certificate of Analysis. Use only pesticides registered with the Department of Agriculture (กรมวิชาการเกษตร), and follow the label. See Pesticide Residues on Cannabis.
Why unlabelled oil is a particular danger
Bonn-Miller and colleagues tested 84 CBD products sold online by 31 companies. They published in JAMA in November 2017. Only about 30% of the products contained an amount within 10% of the label. Several contained THC that the label did not mention.
An unlabelled, untested oil is worse on every point. Its strength, contaminants and solvent are unknown. If something goes wrong, there is no batch record to trace or recall.
What you can do: Never give an untested oil to a person having chemotherapy. If a patient already uses a product, bring its label and test certificate to the oncology team.
14. What a Patient or Carer Should Do
The most useful step on this page is to tell the doctor. ASCO’s 2024 guideline calls for open discussion of cannabis use between doctors and patients, without judgement. It recommends against cannabis as a treatment aimed at the cancer, outside a clinical trial. An oncologist cannot check interactions with a product they do not know about. Write down what you take, how much and when, and bring the list. Questions to ask at the appointment:
- Am I on a checkpoint inhibitor, and does that change your advice?
- Am I on blood thinners, do I have low platelets, or am I at risk of falls?
- Is my white cell count low enough that inhaled plant material is an infection risk?
- Is a licensed cannabinoid medicine available instead?
The legal position in Thailand
Cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. The Notification was issued under the Thai Traditional Medicine Protection Act B.E. 2542 and took effect on 26 June 2025. Flower may only be sold against a PT 33 (ภ.ท.33) prescription from a licensed practitioner, covering at most 30 days. Advertising cannabis flower, including therapeutic claims, is prohibited. Selling flower online is also prohibited. See Advertising and Selling Rules for Cannabis in Thailand.
The Ministerial Regulation on Controlled Herbs (No. 2) B.E. 2569 took effect on 30 April 2026. It tightens the licence rules for cannabis flower. The draft Cannabis Control Act is not law yet. The Cabinet approved the draft on 22 September 2026 and sent it to the House.
Anyone who tells you otherwise, above all a seller who attaches a cancer claim to a product, is breaking the law, and the evidence does not support them either.
What you can do: Buy only through the legal prescription route. Do not buy from a seller who promises to cure cancer. You can report false health claims to the Thai FDA hotline 1556 or to your provincial public health office.
15. Where the Research Is Heading
A trial that could answer whether cannabinoids fight tumours would need five things:
- A standardised product of pharmaceutical quality.
- A delivery route that reaches the concentrations the laboratory work suggests are needed. This is the main unsolved problem.
- The cannabinoid added to standard treatment, not replacing it. Withholding a treatment that works would be unethical.
- Enough patients to detect a realistic difference in survival.
- Survival targets set before the trial starts.
For symptoms, the useful next step is trials that compare cannabinoids directly with modern anti-sickness drugs.
Until then, the honest position is not “cannabis does nothing”. It has a narrow role, recognised in guidelines, for chemotherapy sickness that standard drugs do not control. Its record in cancer pain is disappointing. The evidence elsewhere is insufficient. It has no established role in treating the disease.
If you are a patient or a carer, you are welcome to post in this thread. Tell readers what you tried, what helped, what did not, and what you wish you had known earlier. These accounts help the next frightened reader. Your oncology team knows your cancer stage, your blood counts and your drug list. No one here does. Take what you read here to them. Do not stop, delay or replace treatment because of a forum post, including this one.
This article is part of the Asiannabis Community educational series on medical cannabis. It is general information, not medical advice. Treatment decisions belong with a licensed practitioner. The content is for education in places where medical cannabis is legally permitted.
References for Part 2
- Ramer and Hinz, review of cannabinoids in cancer, British Journal of Cancer, 2022.
- Carkaci-Salli and colleagues, “Cannabinoids as Potential Cancer Therapeutics: The Concentration Conundrum”, Cannabis and Cannabinoid Research 9(4), 2024.
- Study on cannabis, CNR2 and JAK/STAT signalling in T cells, Signal Transduction and Targeted Therapy, 2022.
- Guzmán and colleagues, pilot study of THC in recurrent glioblastoma, British Journal of Cancer, 2006; summary by the US National Cancer Institute.
- Twelves and colleagues, phase 1b trial of nabiximols with temozolomide in recurrent glioblastoma, 2021, and the accompanying editorial.
- ARISTOCRAT trial protocol, BMC Cancer, 2024.
- Cancer Research UK, statement on cannabis and cancer research.
- Johnson and colleagues, alternative medicine and survival, Journal of the National Cancer Institute, 2018.
- Johnson and colleagues, complementary medicine and refusal of conventional treatment, JAMA Oncology, 2018.
- Review of cannabinoid drug interactions, Deutsches Ärzteblatt International, 2023.
- Review of cannabis drug interactions, Journal of Clinical Medicine, 2022.
- Taha and colleagues, cannabis and nivolumab, The Oncologist, 2019.
- Bar-Sela and colleagues, cannabis and immunotherapy outcomes, Cancers, 2020.
- Benedict and colleagues, fungal infections in cannabis users, Emerging Infectious Diseases, 2020.
- Review of the cannabis mycobiome, Frontiers in Microbiology, 2023.
- Bonn-Miller and colleagues, labelling accuracy of CBD products sold online, JAMA, November 2017.
- American Society of Clinical Oncology, guideline on cannabis and cannabinoids in adults with cancer, 2024.
- Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568; Thai Traditional Medicine Protection Act B.E. 2542; Ministerial Regulation on Cannabis or Hemp Extracts B.E. 2569; Ministerial Regulation on Controlled Herbs (No. 2) B.E. 2569.