A brand-name prescription mediation, Paxil (paroxetine hydrochloride) is a common treatment for depression, obsessive-compulsive, panic, premenstrual dysphoric disorder post-traumatic stress disorders. Paxil has the potential to interact with many different medications, including the breast cancer drug tamoxifen.

General Effects of Paxil

In addition to elevating levels of a brain chemical called serotonin in your body that lead to its many benefits, Paxil also has some unwanted effects, one of which is reducing the levels of certain other substances in your body. One such substance is CYP2D6, a type of protein known as an enzyme.

Requirements of Tamoxifen

In your body, tamoxifen breaks down into chemicals that interact with receptors in your breast tissue that are meant for estrogen, interfering with the hormone and reducing the risk of cancers that rely on it. The enzyme CYP2D6 contributes to the breaking down tamoxifen, reports the Pharmacy Times.

Interaction

Since Paxil inhibits the CYP2D6 needed for it to work properly in your body, the combination of the two medications makes tamoxifen less effective, explains the U.S. National Library of Medicine.

Time Frame

The interaction with Paxil and tamoxifen develops gradually making tamoxifen less effective over time, reports the U.S. National Library of Medicine. The rate at which this occurs varies based upon the dosage of each medication.

Risks

Because Paxil makes tamoxifen less effective, the drug increases the likelihood that the anti-estrogen treatment will fail and that breast cancer will return in women who have already developed the disease, warns the Pharmacy Times.

Source:

U.S. National Library of Medicine: PAXIL (paroxetine hydrochloride) tablet, film coated

Mayo Clinic: Paroxetine (Oral Route)

Pharmacy Times: Get to Know an Enzyme-CYP2D6