Background: Cannabidiol (CBD) serves as a promising medicine, with few known adverse effects apart from the potential of drug interactions with the cytochrome P450 system. It has been hypothesized drug interactions may occur with chemotherapeutic agents, but no supporting evidence has been published to date.
Case: A 58-year-old female with a history of bilateral breast carcinoma in remission, was treated with tamoxifen for breast cancer prevention for over 6 years. CBD was instituted to treat persistent postsurgical pain, inadequately managed by alternate analgesics. It was postulated that CBD may diminish tamoxifen metabolism by CYP3A4 and 2D6 to form active metabolite endoxifen, which exerts the anticancer benefits. Endoxifen, tamoxifen, N-desmetyltamoxifen and 4-hydroxytamoxifen levels were collected while the patient chronically received CBD 40 mg/day, and after a 60-day washout. Upon discontinuation of CBD 40 mg/day, it was observed that endoxifen levels increased by 18.75% and N-desmethyltamoxifen by 9.24%, while 4-hydroxytamoxifen remained unchanged.
Conclusion: CBD at a low dose of 40 mg/day resulted in the potential inhibition of CYP3A4 and/or CYP2D6. Patients receiving CBD and interacting chemotherapeutic drugs, such as tamoxifen, require monitoring to identify possible subtherapeutic response to treatment. Further pharmacokinetic studies are required to ascertain the dynamics of this drug interaction.
Keywords: CYP2D6; CYP3A4; cannabidiol; drug interaction; tamoxifen.
That said, it’s important to note that there is no evidence that CBD can treat or cure breast cancer itself.
According to Weiss, some people who use CBD have reported side effects including:
Defining the Terms
Project CBD: “Is CBD Really Non-Psychoactive?”
Mathias Schmucki has also found that talking to other people who currently have or previously had breast cancer is invaluable. A private Facebook group called Fighting Breast Cancer with Cannabis has been helpful in her own journey.
National Center for Complementary and Integrative Health: “Cannabis (Marijuana) and Cannabinoids: What You Need to Know.”
To address the gap, in 2018 the National Council of State Boards of Nursing published Nursing Guidelines for Medical Marijuana, which provides general nursing education and guidance—yet doesn’t discuss drug interactions, specifically. To understand how medical cannabis affects concomitant pharmaceuticals, nurses must have a basic knowledge of the endocannabinoid system (ECS).
The enzymes in the body that synthesize ingested drugs are in the CYP450 family. Cannabidiol (CBD) inhibits or slows the metabolism of the CYP1 family when given 20 minutes before the pharmaceutical. The timing of applying Delta-9-tetrahydrocannabinol (THC) and cannabinol does not slow its metabolism. High concentrations of CBD or THC can boost the production of those enzymes a day later.
How Cannabis Affects Body Systems
When administered sublingually, cannabinoids aren’t immediately processed but neither do they go directly to the brain and heart—like inhaled drugs. Topical administration does not enter the blood stream therefore has no potential for drug interactions.
CYP2D6 metabolizes many opiates, antipsychotics, and antidepressants (both tricyclic antidepressants and selective serotonin reuptake inhibitors). CYP2D6 activates the prodrug tamoxifen, a pharmaceutical treatment for breast cancer. Because CBD inhibits the ID-1 gene, which can reduce breast cancer metastasis, it’s worth studying potential interactions.
Are cannabinoid-drug interactions dangerous? High doses of CBD isolates are the main culprit in issues with adverse drug interactions. Moreover, CBD isolates, unlike whole-plant extracts, generally require higher doses to be effective. A safe rule of thumb is to take cannabis 20 minutes after pharmaceuticals and alert physicians to monitor changes in drug clearance of antiglycemic, antiepileptic, and anticoagulation pharmaceuticals to adjust a patient’s dose accordingly. Further advice can be obtained from a cannabis provider.