Does cannabis interact with Tamoxifen?

"I am being prescribed Tamoxifen and it has many side effects that I was hoping to take cannabis for but I read that it may interfere with the drugs ability to work properly"

Hi there!
It is unlikely.
However, cannabis is a weak competitive inhibitor to many of the liver enzymes in the P450 system; the P450 system metabolizes the vast majority of drugs—prescription and OTC. Tamoxifen and cannabis share two major P450 isoenzymes, CYP3A4 and CYP2C19.

What this means is that THC and/or CBD can block the enzyme(s) responsible for the metabolism of a drug, such as tamoxifen. Therefore, if tamoxifen cannot bind the enzyme that would metabolise it, it would remain in its unmetabolized form, in the circulation.

In the vast majority of cases, the P450 system deactivates the drug and makes it so that the altered drug can be excreted via urination or defecation. However, there are a few drugs that are activated by the enzymes—such is the case with tamoxifen. So, in theory, cannabis might reduce the efficacy of tamoxifen since less of the P450 isoenzyme(s) would be available to activate the tamoxifen. However, that effect would most likely require large doses of cannabis.

And lastly, there was a study suggesting that cannabis revs up (induction), rather than inhibits, the P450 isoenyme CYP1A2. That would mean less tamoxifen in the circulation. However, that study involved Smoking cannabis; polyaromatic hydrocarbons found in all smoke have been shown to induce the CYP1A2 isoenzyme. So that study design was significantly flawed. However, this is yet another reason to avoid smoking cannabis.

I hope this helps!


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