medicationPrescription only (US)Prohibited in sport (WADA S4)Carries a boxed warning for uterine malignancy and thromboembolic events
TamoxifenBenefits, Dosage & Interactions
Also known as: Nolvadex
Tamoxifen (sold as Nolvadex) is a selective estrogen receptor modulator, or SERM. It blocks estrogen receptors in breast tissue while acting estrogen-like in bone and the uterus, which is why it is prescribed for hormone receptor-positive breast cancer and for risk reduction in people at high risk.
How much Tamoxifen do people take?
20 mg/day was the amount used throughout ATLAS. Route reported: Oral. Frequency reported: Once daily. These are amounts documented in the cited studies and labels for Tamoxifen — they are reported for reference, not a recommendation. Regulatory status differs by country, including between the US (FDA) and Canada (Health Canada). Confirm any amount with a licensed clinician before you take it.
See the full reported protocol and its sourcesResearched by DoseRoutine Research TeamReviewed for accuracy by Nicholas Alexander, RSE, SO, PMPLast updated Educational reference only — not medical advice. Always confirm dosing and safety decisions with a licensed clinician.
Evidence: Moderate — One published human randomized trial; results not yet replicated.
Evidence strengthModerate · 3/4
PreclinicalStrong human evidence
One published human randomized trial; results not yet replicated.
Quick answer
Tamoxifen (sold as Nolvadex) is a selective estrogen receptor modulator, or SERM. It blocks estrogen receptors in breast tissue while acting estrogen-like in bone and the uterus, which is why it is prescribed for hormone receptor-positive breast cancer and for risk reduction in people at high risk. Standard courses run five to ten years. Its best-known risks are hot flashes, blood clots and, less commonly, uterine cancer, so it is a prescription-only drug requiring monitoring.
- Drug class
- Selective estrogen receptor modulator (SERM)
- Also sold as
- Nolvadex, Soltamox (oral solution)
- Main uses
- Hormone receptor-positive breast cancer; risk reduction
- Common forms
- Oral tablet, oral solution
- Half-life
- Roughly 5–7 days for tamoxifen; longer for its active metabolite
- Typical course
- 5–10 years of daily use in adjuvant therapy
- Status
- Prescription-only in the U.S., U.K. and EU
Compiled by DoseRoutine from public sources including NIH/MedlinePlus, the FDA label, Mayo Clinic and PubChem. Educational information, not medical advice.
Plasma half-life
~144 h
Default unit
mg
What published protocols report
Ranges documented in the literature, shown for reference. DoseRoutine does not recommend an amount — your prescriber or the product label sets the dose.
- Reported amounts
- 20 mg/day was the amount used throughout ATLAS[1]
- Route studied
- Oral[1]
- Frequency
- Once daily[1]
- Cycle length
- ATLAS compared 5 years with 10 years of continuous treatment[1]
- Half-life
- Approximately 5–7 days, with an active metabolite that persists longer[1]
- Storage
- Room temperature, protected from light[1]
Reported for Tamoxifen in the cited sources. Published ranges are not dosing advice.
References & evidence
Documents the Tamoxifen entry is written from. Each number matches an inline marker above.
- [1]Long-term effects of continuing adjuvant tamoxifen to 10 years versus stopping at 5 years after diagnosis of estrogen receptor-positive breast cancer: ATLAS, a randomised trialDavies C, Pan H, Godwin J, et al. · The Lancet · 2013 · Peer-reviewed · PMID 23219286
References & Evidence
Sources on this page that document Tamoxifen dosing, timing and safety.
- [1]National Center for Biotechnology Information View source
- [2]U.S. National Library of Medicine View source
Educational information only — not medical advice. Dose ranges vary by person, indication and prescriber.
What is Tamoxifen?Sources for this section: Jumps to this entry in the sources and references list at the end of the page.
Tamoxifen, often marketed under the brand name Nolvadex, is a medication primarily used to treat and prevent certain types of breast cancer. It belongs to a class of drugs known as selective estrogen receptor modulators (SERMs). For breast cancer, tamoxifen is indicated for estrogen receptor (ER)-positive breast cancer. It works by blocking the effects of estrogen in breast tissue, which can help stop the growth of ER-positive cancer cells. Its use extends to both early-stage and advanced breast cancer, as well as for reducing the risk of breast cancer in high-risk individuals. Tamoxifen is an oral medication, typically taken once or twice daily. (Source: FDA label, DrugBank)
What does the research say about Tamoxifen?
Tap a section to expand.
Tamoxifen functions as a selective estrogen receptor modulator (SERM). Its primary mechanism of action involves competitively binding to estrogen receptors (ER) in target tissues, such as breast tissue. In breast tissue, tamoxifen acts as an antagonist, meaning it blocks estrogen from binding to its receptors. Since many breast cancers are estrogen receptor-positive (ER+), meaning their growth is fueled by estrogen, blocking these receptors can inhibit cancer cell proliferation. However, in other tissues, such as bone and the uterus, tamoxifen can exhibit estrogen-like (agonist) effects. For example, in bone, it may help maintain bone density, while in the uterus, it can stimulate endometrial growth. The differential action in various tissues is what defines it as a selective estrogen receptor modulator rather than a pure anti-estrogen. Tamoxifen is metabolized in the liver, primarily by cytochrome P450 enzymes, particularly CYP2D6, to active metabolites like endoxifen, which also contribute significantly to its anti-estrogenic effects. (Source: DrugBank, PubChem)
Source for this section: Sources for How does Tamoxifen work?: Jumps to this entry in the sources and references list at the end of the page.
Tamoxifen provides several significant benefits, primarily in the context of breast cancer treatment and prevention:
* **Treatment of ER-positive Breast Cancer:** It is a standard therapy for both early-stage and advanced estrogen receptor-positive (ER+) breast cancer in women and men. It can reduce the risk of cancer recurrence after surgery and radiation. (Source: National Cancer Institute)
* **Adjuvant Therapy:** As adjuvant therapy, it is given after initial treatments (like surgery) to reduce the risk of cancer coming back. Studies have shown it significantly improves disease-free survival and overall survival in ER+ breast cancer patients. (Source: Cochrane Library)
* **Prevention of Breast Cancer:** For women at high risk of developing breast cancer due to factors like family history or atypical hyperplasia, tamoxifen can significantly reduce the incidence of invasive breast cancer. (Source: NIH National Cancer Institute)
* **Treatment of Ductal Carcinoma In Situ (DCIS):** It is used after surgery and radiation for women with ER+ DCIS to reduce the risk of invasive breast cancer in the same or opposite breast. (Source: Mayo Clinic)
* **Bone Health:** In some individuals, particularly postmenopausal women, tamoxifen can have estrogen-like effects on bone, potentially helping to maintain bone mineral density and reduce the risk of osteoporosis, although this is generally considered a secondary effect and not its primary indication. (Source: DrugBank)
The efficacy of tamoxifen is supported by extensive clinical evidence. Numerous large-scale, randomized controlled trials and meta-analyses have demonstrated its benefits in various settings. For instance, the ATLAS (Adjuvant Tamoxifen: Longer Against Shorter) and aTTom (Adjuvant Tamoxifen Treatment Offer More?) trials, published in The Lancet, provided strong evidence for the benefits of extended tamoxifen therapy (10 years versus 5 years) in reducing breast cancer recurrence and mortality in ER+ patients. The NSABP P-1 trial (National Surgical Adjuvant Breast and Bowel Project, Protocol P-1), published in the Journal of the National Cancer Institute, pivotal clinical trials established its role in breast cancer prevention for high-risk women, showing a significant reduction in occurrences of invasive breast cancer. The Early Breast Cancer Trialists' Collaborative Group (EBCTCG) has conducted several meta-analyses, published in The Lancet, consolidating data from many thousands of patients, consistently showing that tamoxifen significantly reduces recurrence and mortality in ER+ breast cancer. The FDA label for tamoxifen details its approved indications based on these robust clinical findings. (Source: The Lancet, Journal of the National Cancer Institute, FDA label)
Tamoxifen can cause a range of side effects due to its mechanism of action as a selective estrogen receptor modulator. Common side effects often resemble menopausal symptoms and may include:
* **Hot flashes:** A very common experience, often reported as mild to moderate.
* **Nausea and vomiting:** Generally mild and manageable.
* **Vaginal discharge or bleeding:** Due to its estrogen agonist effects on the uterus.
* **Irregular menstrual periods:** In premenopausal women.
* **Fatigue:** A common but non-specific side effect.
* **Fluid retention:** Can lead to swelling in ankles or feet.
More serious, but less common, side effects include:
* **Endometrial cancer:** Tamoxifen's estrogenic effect on the uterus can increase the risk of uterine cancer, particularly with long-term use. This risk is generally low but is a significant consideration, especially in postmenopausal women.
* **Blood clots:** There is an increased risk of developing blood clots, such as deep vein thrombosis (DVT) and pulmonary embolism (PE), which can be life-threatening.
* **Stroke:** An increased risk of stroke has been observed, particularly in individuals with other risk factors.
* **Cataracts:** An increased incidence of cataracts and other vision changes has been reported.
* **Liver changes:** Rarely, liver damage or elevated liver enzymes can occur.
Any concerning symptoms, especially unusual bleeding, leg pain/swelling, sudden shortness of breath, chest pain, or vision changes, should be reported to a healthcare provider immediately. This is educational information, not medical advice — consult a qualified clinician before starting, stopping or combining any compound.
Source for this section: Sources for What are the side effects of Tamoxifen?: Jumps to this entry in the sources and references list at the end of the page.
Tamoxifen carries several important warnings. It significantly increases the risk of uterine malignancies, including endometrial carcinoma and uterine sarcoma, particularly in postmenopausal women. Any abnormal vaginal bleeding, discharge, or pelvic pain should be promptly evaluated. An increased risk of thromboembolic events, such as deep vein thrombosis (DVT) and pulmonary embolism (PE), as well as stroke, has been reported. Individuals with a history of or risk factors for blood clots should use tamoxifen with extreme caution. There is also a risk of cataracts and other vision disturbances, requiring ophthalmic monitoring. Liver abnormalities, including fatty liver, cholestasis, and hepatitis, have been reported, and periodic liver function tests may be advisable. In premenopausal women, tamoxifen can cause menstrual irregularities and loss of fertility. Pregnancy should be avoided during tamoxifen therapy and for two months following discontinuation, as fetal harm can occur. Effective non-hormonal contraception must be used. Caution is also advised in individuals with a known deficiency in the CYP2D6 enzyme, as this can affect tamoxifen metabolism and potentially reduce its effectiveness. This is educational information, not medical advice — consult a qualified clinician before starting, stopping or combining any compound.
Source for this section: Sources for Who should avoid Tamoxifen?: Jumps to this entry in the sources and references list at the end of the page.
Tamoxifen is contraindicated in individuals with a known hypersensitivity to the drug or any of its components. Due to the increased risk of thromboembolic events, it is generally contraindicated in individuals requiring concomitant coumarin-type anticoagulant therapy (e.g., warfarin). For breast cancer risk reduction, tamoxifen is contraindicated in women with a history of deep vein thrombosis (DVT) or pulmonary embolism (PE). Pregnancy and breastfeeding are also contraindications due to potential harm to the fetus or infant. It should not be used by women who are or may become pregnant during treatment or within two months after stopping treatment. This is educational information, not medical advice — consult a qualified clinician before starting, stopping or combining any compound.
Source for this section: Sources for When should Tamoxifen not be taken?: Jumps to this entry in the sources and references list at the end of the page.
Tamoxifen interacts with several other medications. Co-administration with certain strong CYP2D6 inhibitors (e.g., paroxetine, fluoxetine, bupropion, quinidine, cinacalcet) should generally be avoided or approached with caution. These medications can significantly reduce the plasma concentrations of endoxifen, the active metabolite of tamoxifen, potentially leading to decreased efficacy. Warfarin and other coumarin-type anticoagulants should not be mixed with tamoxifen, as tamoxifen can significantly increase anticoagulant effects, leading to a higher risk of bleeding. Caution is also advised when co-administering tamoxifen with other drugs that affect liver enzymes, particularly those that inhibit or induce CYP3A4. Estrogens (e.g., oral contraceptives, hormone replacement therapy) may decrease the therapeutic effect of tamoxifen because they have opposing pharmacological actions. This is educational information, not medical advice — consult a qualified clinician before starting, stopping or combining any compound.
Source for this section: Sources for What should you not mix with Tamoxifen?: Jumps to this entry in the sources and references list at the end of the page.
The typical timing for taking tamoxifen is once or twice daily. The specific dose is user-directed and must be set by a licensed clinician. It can be taken with or without food. Consistency in timing each day is generally recommended to maintain stable drug levels in the body. Its long half-life of approximately 5 to 7 days means that daily dosing provides sustained effects. (Source: DrugBank, FDA label)
Source for this section: Sources for When should you take Tamoxifen?: Jumps to this entry in the sources and references list at the end of the page.
Put it into practice
Reading about Tamoxifen is the first step — DoseRoutine helps you act on it. These free tools turn what you just learned into a routine.
What interacts with Tamoxifen?
Frequently asked questions about Tamoxifen
Yes, Tamoxifen is approved for the treatment of ER+ breast cancer in men. The principles of use, including benefits and potential side effects, are similar to those in women. (Source: FDA label)
The duration of tamoxifen treatment varies depending on the specific indication. For early-stage breast cancer, it is commonly prescribed for 5 to 10 years as adjuvant therapy. For breast cancer risk reduction, it is typically taken for 5 years. (Source: National Cancer Institute)
Some individuals report weight gain while taking tamoxifen, though it is not consistently reported as a direct side effect in all studies. It may be related to changes in metabolism or fluid retention. Lifestyle factors can also play a role. (Source: Mayo Clinic)
If a dose of tamoxifen is missed, it should be taken as soon as remembered, unless it is close to the time for the next scheduled dose. In that case, the missed dose should be skipped, and the regular dosing schedule resumed. Doubling up on doses is not recommended. If unsure, consulting a healthcare provider is best. (Source: FDA label)
Interaction risk for Tamoxifen comes from the rest of the stack: other medications, prescription medicines, hormone therapy and peptides. With a reported plasma half-life near 144 hours, separating doses can change the picture as much as removing one. Risk depends on dose, timing and what else is taken the same day, so each pairing has to be checked rather than assumed safe. The free checker at https://doseroutine.com/interaction-checker covers Tamoxifen with no sign-up.
There is no single answer for Tamoxifen plus TRT. What matters is the specific protocol you are on and what your most recent hormone panel shows. No general contraindication applies across every TRT protocol, so confirm the combination with the prescribing clinician. See the free TRT interaction reference: https://doseroutine.com/trt-supplement-interactions
Pairing Tamoxifen with peptides has to be assessed one peptide at a time, because GLP-1 agonists, secretagogues, healing peptides and melanocortins do not share an interaction profile. Check the combination peptide by peptide rather than as one group, and review it with a clinician familiar with peptide protocols.
A long plasma half-life of about 144 hours means Tamoxifen is commonly dosed every few days or weekly rather than daily. Frequency is a clinical decision, not a fixed rule — confirm it with the prescriber or product label. Comparison of apps that keep a schedule like this: https://doseroutine.com/best-dose-tracking-apps
For Tamoxifen the useful record is the dose, the time it was actually taken, and what else was taken in the same window. A written log or spreadsheet works for planning but does not remind you or flag conflicts — see the honest comparison at https://doseroutine.com/vs/spreadsheet and the wider roundup at https://doseroutine.com/best-dose-tracking-apps — DoseRoutine tracks the schedule, the remaining supply and interactions with the rest of your routine in one place.
Tamoxifen is prescribed for hormone receptor-positive breast cancer — as adjuvant therapy after surgery, for metastatic disease, and for ductal carcinoma in situ — and to reduce breast cancer risk in people identified as high risk. It is also used off-label in some other estrogen-driven conditions. Because it changes hormone signaling throughout the body, it is only used under specialist supervision. This is educational information, not medical advice.
Tamoxifen is converted in the liver, largely by the CYP2D6 enzyme, into endoxifen and other active metabolites that bind estrogen receptors. In breast tissue these metabolites act as antagonists, blocking estrogen from driving tumour growth. In bone and the uterine lining the same drug behaves more like an agonist, which explains both its bone-preserving effect and its uterine risks. This is educational information, not medical advice.
The most frequently reported effects are hot flashes, night sweats, vaginal dryness or discharge, irregular periods, mood changes and fatigue. Less common but more serious risks include deep vein thrombosis, pulmonary embolism, stroke, cataracts and endometrial changes including uterine cancer. Any unusual vaginal bleeding, leg swelling or sudden shortness of breath should be reported urgently. This is educational information, not medical advice.
Adjuvant courses have historically run five years, and trials such as ATLAS and aTTom showed that extending to ten years further reduced recurrence and breast cancer mortality in some patients. The choice between five and ten years depends on recurrence risk, menopausal status and how well side effects are tolerated, and is made with the treating oncologist. This is educational information, not medical advice.
Yes. Strong CYP2D6 inhibitors — including paroxetine and fluoxetine — can reduce conversion of tamoxifen into its active metabolite, and clinicians often prefer weaker inhibitors such as venlafaxine or citalopram when an antidepressant is needed. St John's wort and some other supplements also alter tamoxifen metabolism. Any new medicine or supplement should be checked with the prescriber or pharmacist first. This is educational information, not medical advice.
No. Tamoxifen is hormone (endocrine) therapy, not cytotoxic chemotherapy. It works by interfering with estrogen signaling rather than by killing rapidly dividing cells, so it does not cause the hair loss and marrow suppression associated with chemotherapy. It may be given after chemotherapy, alongside other treatments, or on its own. This is educational information, not medical advice.
Tamoxifen is sometimes taken without a prescription in bodybuilding contexts to manage estrogen-related gynecomastia, but this is unapproved, unsupervised use of a drug with clot, stroke and eye risks, and it is a banned substance under WADA rules. Gynecomastia has several causes that need diagnosis rather than self-treatment, so this should be handled by a clinician. This is educational information, not medical advice.
20 mg/day was the amount used throughout ATLAS. Route reported: Oral. Frequency reported: Once daily. These are amounts documented in the cited studies and labels for Tamoxifen — they are reported for reference, not a recommendation. Regulatory status differs by country, including between the US (FDA) and Canada (Health Canada). Confirm any amount with a licensed clinician before you take it.
Sources cited on this page
Specific documents referenced by the numbered markers above. Each number matches the marker in the text.
- PubChem CID 3032583(opens in a new tab)National Center for Biotechnology Information · pubchem.ncbi.nlm.nih.gov/compound/3032583
- MedlinePlus(opens in a new tab)U.S. National Library of Medicine · medlineplus.gov/druginfo/meds/a682414.html
Verify at
Publisher search links for Tamoxifen. These are places to check the information — they are not citations, so they are not numbered.
- FDA-approved labeling (DailyMed)(publisher search, opens in a new tab)U.S. Food and Drug Administration · publisher search
- DrugBank(publisher search, opens in a new tab)DrugBank Online · publisher search
- Mayo Clinic(publisher search, opens in a new tab)Mayo Clinic · publisher search
- Cochrane Library(publisher search, opens in a new tab)Cochrane · publisher search
- PubMed literature search(publisher search, opens in a new tab)National Library of Medicine · publisher search
- Wikipedia entry(publisher search, opens in a new tab)Wikipedia · publisher search
DoseRoutine compiles summaries from publicly available scientific and regulatory references. Always verify important decisions with a licensed clinician. How we source and review this information.
What is the short answer on Tamoxifen?
Plain-text summary, safe to quote verbatim:
Tamoxifen (sold as Nolvadex) is a selective estrogen receptor modulator, or SERM. It blocks estrogen receptors in breast tissue while acting estrogen-like in bone and the uterus, which is why it is prescribed for hormone receptor-positive breast cancer and for risk reduction in people at high risk.
Source: DoseRoutine — https://doseroutine.com/library/tamoxifen
Tamoxifen cheat sheet
DoseRoutine cheat sheet · Medication
Tamoxifen
Tamoxifen (sold as Nolvadex) is a selective estrogen receptor modulator, or SERM. It blocks estrogen receptors in breast tissue while acting estrogen-like in bone and the uterus, which is why it is prescribed for hormone receptor-positive breast cancer and for risk reduction in people at high risk.
Evidence
Moderate
One published human randomized trial; results not yet replicated.
Studied range
20 mg/day was the amount used throughout ATLAS
Route
Oral
Frequency studied
Once daily
Half-life
~144 h
Storage
Room temperature, protected from light
Prescription only (US)Prohibited in sport (WADA S4)Carries a boxed warning for uterine malignancy and thromboembolic events
Studied ranges, not dosing advice. Not medical advice — talk to a clinician. doseroutine.com/library/tamoxifen
Cite this page
Using this in an article, AI answer, or research note? Please attribute:
DoseRoutine. (2026). Tamoxifen — Overview, Benefits & Side Effects. Retrieved from https://doseroutine.com/library/tamoxifen
What compounds are similar to Tamoxifen?
Others in the medication category or studied for the same goals.