CBD, Cannabis, and Chemotherapy Drug Interactions

Most questions about cannabis and cancer treatment run into missing data. Drug interactions are the exception. One cannabidiol product is an FDA-approved prescription medicine with a full label. That label names the liver enzymes cannabidiol affects, and says what to do about each.

That does not make an unregulated CBD oil the same as the medicine. It does mean the pharmacology is documented, and the direction of each interaction is known.

Most cancer drugs are broken down by a family of liver enzymes called cytochrome P450, often shortened to CYP. Other drugs are cleared by a second family called UGT. A third system, a pump called P-glycoprotein or P-gp, controls how much of a swallowed drug is absorbed from the gut.

If something slows one of those systems, the drug it handles builds up. That means more effect and more side effects from the same dose. If something speeds a system up, the drug clears faster and may stop working.

NCI states directly that cannabinoids are known to interact with the liver cytochrome P450 system.

Epidiolex is a purified cannabidiol solution. Here is what its approved label reports.

Cannabidiol slows some enzymes down:

• CYP2C19. Cannabidiol is a moderate inhibitor. Levels of drugs cleared this way rise, raising the risk of side effects.
• CYP1A2. Cannabidiol is a weak inhibitor. Named examples of affected drugs are theophylline and tizanidine.
• CYP2C8. The label warns of clinically significant interactions and tells prescribers to consider easing back the affected drug.
• UGT1A9. Cannabidiol inhibits this enzyme, so exposure to drugs cleared by it can rise.

One enzyme goes both ways. Cannabidiol both induces and inhibits CYP2B6. At the amounts studied for the prescription product, no clinically significant change turned up. Beyond those amounts nobody has measured it, so the effect is simply unknown.

Everything in this section comes from a prescription label written for prescribers, and the amounts behind it were fixed by study protocols. None of it is a plan for anyone at home. If you use cannabis or CBD, tell your oncology team the product and how much you take, so they can watch for interactions.

A prodrug problem runs in the opposite direction. Some drugs are inactive until the body converts them. Clopidogrel, a blood thinner, is the label's named example. Because cannabidiol inhibits CYP2C19, it can lower the amount of active metabolite formed, which may make the drug work less well. For those drugs the label points prescribers in the opposite direction from what most people would expect: the amount may need to go up rather than down. That is a calculation for your prescriber, not a change to make yourself.

Absorption is affected too. Cannabidiol raises exposure to swallowed P-glycoprotein substrates. With everolimus, a drug used in several cancers, the increase was about 2.5-fold. The label tells prescribers to measure everolimus blood levels and rework the amount accordingly. It names sirolimus, tacrolimus, and digoxin as other oral P-gp substrates that may rise.

Rifampin runs the other way. A strong CYP3A4 and CYP2C19 inducer lowered cannabidiol levels by about 32%, and its active metabolite by about 63%.

The label carries a separate warning about the liver. Cannabidiol combined with valproate raises the rate of liver enzyme elevations. If that happens, the label says to consider stopping or reducing either drug.

It also states plainly that the data are insufficient elsewhere. The risk of taking cannabidiol with other liver-toxic drugs has not been assessed. Many cancer treatments are hard on the liver. That gap is the honest reason to ask an oncology pharmacist rather than guess.

One study is worth knowing because it is often cited in both directions.

Twenty-four people with cancer took part. Twelve received intravenous irinotecan. Twelve received docetaxel. Three weeks later each had the same drug again. This time they also took medicinal cannabis as an herbal tea, for 15 days straight, starting 12 days before the second dose.

Cannabis did not significantly change exposure to either drug. It did not change clearance either. NCI adds the caveat that matters. An herbal tea may not reproduce what happens with inhaled cannabis. Nor what happens when fat-soluble cannabinoids are swallowed.

So the finding is reassuring for those two drugs by that route. It does not generalize to concentrated CBD oil.

NCI is direct about this. Highly concentrated THC or CBD oil extracts are illegally promoted as cancer cures. No clinical trial has evaluated them for anticancer activity or for safety.

The interaction risk follows from the concentration. CBD inhibits certain cytochrome P450 enzymes. Concentrated oils taken with treatments cleared by those enzymes could raise toxicity. They could also cut effectiveness.

There is also a labeling problem. NCI notes concerns about the reliability of cannabis testing labs. Some have been accused of inflating the cannabinoid content they report. One study found major swings in reported THC content. The cause was inconsistent sample preparation and testing methods.

In practice, that means the amount on the bottle may not be the amount you take.

The useful conversation is specific, not general. Bring:

• The product name, the stated CBD and THC content per dose, and the number of doses per day.
• Your full medicine list, including anything over the counter.
• The route you use, since swallowed, inhaled, and under-the-tongue products behave differently. Oral bioavailability of cannabis is low and variable, at 6% to 20%, and peak THC levels arrive 1 to 6 hours after swallowing. Inhaled cannabinoids peak in 2 to 10 minutes.

Then ask three questions:

• Is any drug I take cleared by CYP2C19, CYP1A2, CYP2C8, CYP2B6, or UGT1A9?
• Am I on any oral P-glycoprotein substrate, such as tacrolimus, sirolimus, digoxin, or everolimus?
• Should any drug level be monitored, and when?

An oncology pharmacist can answer all three from your medication list in a few minutes. That is a far better use of the question than a general yes or no.

Two related pages may help. For appetite and eating questions, see nutrition during treatment. For the team that usually handles symptom control, see palliative care.

• Call 911 or go to an emergency department if a racing heartbeat, fainting or chest pain follows a cannabis or CBD product, or the person cannot be roused.
• Call your care team the same day if your eyes or skin look yellow, your urine darkens, or you ache under the right ribs. Cannabidiol can raise liver enzymes, and plenty of cancer drugs already tax the liver.
• Call your care team within a day or two if heavy drowsiness, dizziness, hallucinations or paranoia start after you begin a product. NCI lists all of these among reported effects. Bring the bottle so the dose can be read off the label.

• National Cancer Institute, Cannabis and Cannabinoids PDQ, patient version; accessed August 11, 2026
• DailyMed, Epidiolex (cannabidiol) FDA label, published June 15, 2026; accessed August 6, 2026
• National Cancer Institute, Cannabis and Cannabinoids PDQ, health professional version, updated May 13, 2025; accessed August 6, 2026