Drug Herb Interactions: Tamoxifen by M.M. Van Benschoten, OMD
The primary clinical use of Tamoxifen is for prevention and treatment of estrogen receptor positive breast cancer. Tamoxifen acts as an antagonist, blocking the binding of estrogen to the receptor site, reducing DNA synthesis and cancer growth. The research literature contains many references to herbal medicines that interact with Tamoxifen by enhancement or inhibition of antitumor effects. Tamoxifen is metabolized by the liver into the active metabolite, 4-hydroxytamoxifen. This transformation is induced by cytochrome P450 CYP2D6 and CYP3A4 enzymes. Herbs interacting with these liver enzymes may reduce clinical effectiveness of the drug.
CYP2D6 inhibit
alpinia officinarum (gao liang jiang), epimedium (yin yang huo), ginkgo leaf (yin xing ye), piper nigrum (hu jiao), punica granatum (shi liu pi), rhodiola (hong jing tian)
CYP3A4 inhibit
andrographis (chuan xin lian), agaricus (ji song rong), bamboo (zhu ru), camellia (lu cha), cimicifuga (sheng ma), curcuma longa (jiang huang), curcuma zedoria (e zhu), epimedium (yin yang huo), evodia (wu chu yu), fennel (xiao hui xiang), ginger (sheng jiang), ginseng (ren shen), grape seed (pu tao zi), licorice (gan cao), rhodiola (hong jing tian), schizandra (wu wei zi), valerian (jie cao)
All the above species are contraindicated in patients taking Tamoxifen, as they can inhibit the transformation of the drug to its bioactive state. Cancer patients often self prescribe curcumin/tumeric and mushroom preparations based on their anticancer effects without considering interactions with prescription medications.
Tamoxifen enhance
camellia (lu cha), cimicifuga (sheng ma), linum (flax seed), morus branch (sang zhi), silybum (shui fei ji)
These species can increase bioavailability of Tamoxifen, and inhibit growth of Tamoxifen resistant cancer cells. Camellia and cimicifuga are CYP3A4 inhibitors, so their use should be avoided.