Tamoxifen (Nolvadex) is the foundational antiestrogen, FDA-approved in 1977 and one of the most-prescribed cancer drugs ever made. It is a selective estrogen receptor modulator (SERM): a blocker in breast tissue (the basis for hormone-receptor-positive breast cancer treatment and prevention) and a partial activator in bone, liver, and uterus. The active metabolite endoxifen, generated by the liver enzyme CYP2D6, is roughly a hundred times more potent than the parent drug, so people with low CYP2D6 activity may respond poorly. Off-label, it is widely used as post-cycle therapy by anabolic steroid users to restore natural testosterone production. Side effects include hot flashes and a small increase in endometrial cancer risk. Not stocked by Kodiac. This monograph is provided for research and educational reference.
Intrigue 0–100 blends mechanism novelty, evidence strength, and translational potential. Kodiac editorial, not peer-reviewed.
Selective estrogen receptor modulator (triphenylethylene)
A triphenylethylene SERM; the foundational antiestrogen for ER-positive breast cancer and an off-label tool for SARM/AAS post-cycle therapy.
Abstract
Tamoxifen ((Z)-2-[4-(1,2-diphenyl-1-butenyl)phenoxy]-N,N-dimethylethanamine; CAS 10540-29-1; molecular formula C26H29NO; molecular weight 371.51) is a triphenylethylene SERM developed at ICI Pharmaceuticals (now AstraZeneca) and approved by the FDA in 1977 under the trade name Nolvadex. The compound is a tissue-selective estrogen receptor modulator: antagonist in breast tissue (the basis for breast cancer indications) and partial agonist in bone, liver, and uterus. The active metabolite endoxifen (4-hydroxy-N-desmethyltamoxifen) generated via CYP2D6 is approximately 100-fold more potent than tamoxifen at ER binding and contributes substantially to clinical activity; CYP2D6 polymorphism affects clinical response. Plasma half-life is approximately 5 to 7 days for parent compound. Approved indications include adjuvant treatment of ER-positive breast cancer (premenopausal and postmenopausal), metastatic breast cancer, and breast cancer prevention in high-risk patients. Off-label use in male hypogonadism, gynecomastia treatment, and post-cycle therapy following SARM or anabolic steroid use is common. Adverse events include endometrial cancer (rare), thromboembolism, and hot flashes. Used as the canonical SERM in mechanism studies.
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