Found this while surfing, never seen it mentioned on the forum.

Drugs that can block effects of Tomaxifen

Medicines to avoid while taking tamoxifen

In the list below, the medications under the headings “Strong Inhibitors” and “Moderate Inhibitors” can inhibit CYP2D6 and interfere with the effectiveness of tamoxifen. The medications under the heading “Not Inhibitors” do not block the CYP2D6 enzyme and will not interfere with tamoxifen treatment.

This list is incomplete and subject to change over time. Use it as a starting place and ask your doctor if any medications you are taking or that are recommended to you are compatible with tamoxifen.

Strong Inhibitors
Generic Names Brand Names

Bupropion Wellbutrin
Fluoxetine Prozac
Paroxetine Paxil
Quinidine Cardioquin

Moderate Inhibitors
Generic Names Brand Names

Duloxetine Cymbalta
Sertraline Zoloft
Diphenhydramine Benadryl
Thioridazine Mellaril
Amiodarone Cordarone
Trazodone Desyrel
Cimetidine Tagamet

SSRIs and SNRIs That Are Not Inhibitors
Generic Names Brand Names

Venlavaxine Effexor
Citalopram Celexa
Escitalopram Lexapro