Nolvadex (Tamoxifen) 10 mg / 20 mg Tablets: Prescription Selective Estrogen Receptor Modulator for ER+ Breast Cancer Adjuvant Therapy and High-Risk Prevention

Reviewed by the Lucas Clinic Medical Team | Updated May 2026

Nolvadex (tamoxifen citrate) is a selective estrogen receptor modulator (SERM) — a class of compounds that interact with estrogen receptors and produce either estrogenic agonist or antagonist effects depending on the tissue. In breast tissue, tamoxifen is a competitive antagonist at estrogen receptors (ER), blocking estrogen binding and producing an anti-estrogenic effect that inhibits ER+ breast tumour growth. This makes it highly effective in treating estrogen receptor-positive (ER+) breast cancer — the most common type, representing approximately 70–80% of all breast malignancies. In contrast, tamoxifen has partial estrogenic agonist activity in the uterus (increasing endometrial proliferation — raising endometrial cancer risk), bone (protective — reducing osteoporosis), and liver (affecting coagulation — increasing VTE risk). This tissue-specific duality is the hallmark of the SERM class.

Tamoxifen was first approved by FDA in 1977 for metastatic breast cancer, making it one of the oldest established targeted cancer therapies. It has since received approvals for: adjuvant treatment of early ER+ breast cancer (post-surgery, 5 years standard; 10 years in high-risk patients — the aTTom and ATLAS trials showed ongoing benefit from extending to 10 years); reduction of breast cancer risk in women at high risk (chemoprevention); treatment of ductal carcinoma in situ (DCIS) after surgery; and male breast cancer. Tamoxifen remains standard of care for premenopausal women with HR+ early breast cancer, for whom aromatase inhibitors cannot be used as monotherapy.

What Is Tamoxifen?

Tamoxifen is a triphenylethylene derivative that functions as a broad estrogen receptor ligand. Its pharmacological complexity lies in its active metabolites — tamoxifen itself has relatively low ER affinity, but is metabolised by CYP2D6 (and to a lesser extent CYP3A4) to 4-hydroxytamoxifen (4-OHT) and endoxifen, both of which have 100-fold greater ER affinity than tamoxifen parent compound. Endoxifen is now considered the primary pharmacologically active metabolite responsible for tamoxifen's clinical efficacy.

This metabolic pathway has critical clinical implications: patients who are CYP2D6 poor metabolisers (a pharmacogenomic trait affecting approximately 7–10% of people of European ancestry) produce significantly less endoxifen and may have reduced tamoxifen efficacy. Strong CYP2D6 inhibitors — including the antidepressants paroxetine (Paxil) and fluoxetine (Prozac) — substantially reduce endoxifen levels and should be avoided during tamoxifen therapy wherever possible. When antidepressant therapy is required for tamoxifen-related menopausal symptoms, venlafaxine or citalopram (which have minimal CYP2D6 inhibition) are preferred.

Prescription vs. Over-the-Counter Status

Tamoxifen is prescription-only in the US, UK, EU, and all comparable markets. It is not available OTC under any approved formulation. Generic tamoxifen is widely available. Like anastrozole and all endocrine therapies for breast cancer, tamoxifen is managed by oncologists and breast cancer teams, though ongoing stable adjuvant prescriptions may be transferred to primary care with appropriate monitoring plans. Illicit use by male bodybuilders (to counter gynecomastia from anabolic steroid use) is prevalent online but constitutes off-label, non-prescribed use; purchasing tamoxifen without a valid prescription is illegal in the US and UK.

Available Strengths and Forms

Tamoxifen is available in two tablet strengths: 10 mg and 20 mg. The standard dose for breast cancer treatment and adjuvant therapy is 20 mg once daily, taken at the same time each day. The same dose is used for chemoprevention in high-risk women. Some regimens use 10 mg twice daily as an alternative. Generic tamoxifen is widely available in both strengths from multiple manufacturers. Tamoxifen tablets are taken orally, with or without food. Consistent daily intake at the same time maximises steady-state plasma levels. Half-life of tamoxifen is approximately 5–7 days, meaning it takes approximately 4–8 weeks to reach steady state and a similar period after stopping for plasma levels to fall substantially.

Price of Nolvadex (Tamoxifen)

Generic tamoxifen 20 mg is among the most affordable oncology prescriptions. At US pharmacies, a 30-tablet supply (one month) costs approximately $20–$50 without insurance; GoodRx coupons can reduce this below $15 at some pharmacies. Brand Nolvadex is no longer widely available. In the UK, tamoxifen is dispensed on NHS prescription. Given its low acquisition cost and decades of evidence, tamoxifen is considered exceptionally cost-effective in the adjuvant breast cancer setting — representing remarkable value given the 5–10 year recurrence risk reduction it provides. AstraZeneca patient assistance programmes and oncology charities offer support for patients who face barriers to access.

Frequently Asked Questions

What is the most important drug interaction with tamoxifen?

The most clinically important drug interaction is with strong CYP2D6 inhibitors — particularly the antidepressants paroxetine and fluoxetine, which can reduce endoxifen (tamoxifen's active metabolite) plasma levels by 60–70%. This reduction is associated with potentially reduced anti-tumour efficacy in observational studies, and is a well-recognised pharmacokinetic interaction. Patients taking tamoxifen who require antidepressant therapy for depression or hot flushes should use agents with minimal CYP2D6 inhibition: venlafaxine, desvenlafaxine, or escitalopram/citalopram. Bupropion also inhibits CYP2D6 significantly and should be avoided. Oncology pharmacists and prescribers should review all concomitant medications at each consultation for CYP2D6 interactions.

Does tamoxifen cause endometrial cancer?

Yes — tamoxifen has partial estrogenic agonist activity at uterine estrogen receptors, stimulating endometrial proliferation. Long-term use is associated with a small but established increased risk of endometrial hyperplasia and endometrial carcinoma (approximately 2–3-fold relative risk increase over 5 years, translating to an absolute risk of approximately 2–4 per 1,000 women per year from baseline approximately 1 per 1,000). Any abnormal vaginal bleeding, discharge, or pelvic pain in women taking tamoxifen should be evaluated promptly with transvaginal ultrasound to assess endometrial thickness, and endometrial biopsy if indicated. Annual gynaecological review is recommended during tamoxifen use. This endometrial risk is one reason aromatase inhibitors are preferred in postmenopausal women — AIs do not carry endometrial stimulation risk.

Can tamoxifen be used in men?

Yes. Male breast cancer, though rare (approximately 1% of all breast cancer), is predominantly ER+ and tamoxifen is the standard systemic endocrine therapy. Dosing and adjuvant duration recommendations are similar to female breast cancer, though evidence in men is based primarily on retrospective studies and extrapolation from female data. Tamoxifen is also widely used off-label by male anabolic steroid users to counteract estrogenic side effects (particularly gynecomastia) — this use is not sanctioned by any regulatory body or medical guideline, carries risks of thromboembolic disease (tamoxifen raises VTE risk through liver effects on coagulation factors), and should not be self-administered without medical supervision.

← Back to Hormone Therapy Medications