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Lexicon
PCT & hormone control

Tamoxifen (Nolvadex)

A SERM that blocks estrogen directly at breast tissue, which is why it is the first choice against gynecomastia.

2 min read 5 sources

To the product To the sources
White tablet bottle with a black TPD label for tamoxifen on a white podium, next to it a large blue 3D molecule in a bright lab
Compound approved · since 1977 according to ChEMBLLab tested in the shop

Summary

At a glance

Sources checked on 26 Oct 2026

Gynecomastia with tamoxifen
10 %

over 48 weeks, 73% without tamoxifen1

Breast pain with tamoxifen
6 %

same trial, 39% without tamoxifen1

Risk in the meta-analysis
-82 %

for gynecomastia and breast pain, 9 trials2

Complete resolution
90.1 %

81 men with gynecomastia of unknown cause, no control group3

Adverse events
35 %

37% without tamoxifen, 69% with anastrozole1

  • Tamoxifen is an estrogen receptor modulator. It does not lower the estrogen level but slows the effect at the ERα receptor.5
  • In a controlled trial with 114 men on an antiandrogen 73 percent developed gynecomastia without tamoxifen, 10 percent with tamoxifen.1
  • The aromatase inhibitor anastrozole reached 51 percent in the same trial. A meta-analysis of 9 trials confirms the lead of tamoxifen.12
  • Adverse events were not more common with tamoxifen than without. Testosterone rose, free testosterone stayed the same.1
  • Tamoxifen is an approved compound. Everything here is the state of research, not a recommendation.5
Profile

Updated May 2026

Class
SERM

estrogen receptor modulator

Use
Oral

daily

Half-life
long

several days

WADA
Banned

category S4

Sizes
10 to 20 mg

2 sizes in the shop

Contents 14 sections
  1. At a glance
  2. What is Tamoxifen?
  3. How it works
  4. How to take it
  5. Dosing
  6. What the research shows
  7. Research status
  8. Side effects
  9. Practice
  10. Storage
  11. Related compounds
  12. FAQ
  13. Glossary
  14. Sources

What is Tamoxifen?

Tamoxifen, known as Nolvadex, is a selective estrogen receptor modulator (SERM). In medicine it is best known from breast cancer therapy. In the performance corner it is used for PCT and especially against gynecomastia.

The difference from aromatase inhibitors: Tamoxifen does not lower the estrogen level but blocks the estrogen effect where it causes trouble, mainly at breast tissue.

Comparison

Blocking or lowering estrogen

Tamoxifen and clomiphene act at the receptor, aromatase inhibitors lower the production of estrogen. Tap a card to open its entry.

Receptor modulator Tamoxifen (Nolvadex) ERα
Receptor modulator Clomid (Clomiphene) ERα Aromatase inhibitor Arimidex (Anastrozole) Aromatase Aromatase inhibitor Letrozole (Femara) Aromatase

According to ChEMBL: tamoxifen and clomiphene modulate estrogen receptor alpha, anastrozole, letrozole and exemestane inhibit aromatase (CYP19A1). 5

How it works

Tamoxifen acts differently depending on the tissue. At breast tissue it blocks the estrogen receptor, which is exactly what stops or slows the formation of breast gland tissue (gyno). At the hypothalamus it acts similarly to Clomifene and drives the body's own testosterone production via more LH and FSH.

So Tamoxifen covers two uses: gyno control and PCT support.

Where it acts

Where tamoxifen acted in studies

Tap a point or a card. Colour = estrogen receptor ERα, grey = observed in studies.

  1. Breast gland

    Gynecomastia in 10 instead of 73 percent, breast pain in 6 instead of 39 percent1

    ERα
  2. Hormones in the blood

    Testosterone rose, free testosterone stayed the same because SHBG rose1

    Observed in trials
  3. Prostate

    PSA drop on the antiandrogen in 97 percent, as without tamoxifen1

    Observed in trials

Targets in the ChEMBL database5

Compound CHEMBL83: one recorded mechanism, Modulator.

Estrogen receptor alpha
Gene
ESR1
UniProt
P03372
ChEMBL
CHEMBL206
Action
Modulator
Blue molecules dock onto translucent bubble-like glass cells under a bright cone of light
Tamoxifen occupies the estrogen receptor in breast tissue. Symbolic image.

How to take it

Oral, once daily.

  • One dose per day is enough given the long half-life.
  • With some water, with or without food.
  • For acute gyno it is dosed higher and shorter, for PCT lower over several weeks.
White tablet bottle with a black TPD label for tamoxifen, next to it three tablets and a glass of water
Once a day with some water, with or without food. Symbolic image.

Dosing

Dosing

Dosing, titration and syringe

Loading the dosing information …

  • Getting started and stepping up, drawn as a staircase
  • Units on the U-100 syringe for every step
  • How long one vial lasts

What the research shows

A controlled study (Boccardo, 2005) examined the prevention of gynecomastia under an estrogen-driving therapy. Result: in the placebo group 73 percent developed gyno, with Tamoxifen 20mg only 10 percent. The aromatase inhibitor anastrozole was clearly weaker in this study (51 percent).

That is a strong argument for why Tamoxifen is the first choice for acute gyno and not an aromatase inhibitor.

Study results

What the trials measured

Interactive: switch tabs, tap points, show or hide arms. Arms are named relative to each other, no dosing advice.

Breast changes on an antiandrogen1

Share of men in percent, 114 men with prostate cancer, all received bicalutamide

Gynecomastia1
No protection (placebo) 73%
Tamoxifen 10%
Anastrozole (comparator) 51%
Breast pain1
No protection (placebo) 39%
Tamoxifen 6%
Anastrozole (comparator) 27%
PSA at least halved1 the cancer treatment remained effective
No protection (placebo) 97%
Tamoxifen 97%
Anastrozole (comparator) 83%

Values from the abstract. Bicalutamide is an antiandrogen that often triggers gynecomastia through more estrogen action. Sexual function showed no major differences up to month 6.

Relative risk versus no prevention2

1.0 means as common as without prevention, lower is better

Gynecomastia2
Tamoxifen 0.18-fold
Breast radiotherapy (comparator) 0.48-fold
Breast pain2
Tamoxifen 0.18-fold
Breast radiotherapy (comparator) 0.66-fold

Values from the abstract, 9 randomised trials with bicalutamide. Anastrozole showed no significant benefit and therefore has no bar.

Gynecomastia of unknown cause3

Share with complete resolution, 81 men over 10 years in a breast clinic

Complete resolution3
Tamoxifen 90.1%

Observational study without a control group. 73 of 81 men responded completely, 2 of the other 8 had surgery.

Frosted glass sculpture of a breast in a glass showcase on a white plinth
Trials on preventing gynecomastia. Symbolic image.

Research status

Where the research stands

Tamoxifen has been approved since 1977. On ClinicalTrials.gov 685 trials list tamoxifen among their interventions, almost all on breast cancer. Shown are the registered phase 3 trial on gynecomastia and three large breast cancer trials. As of 26 Oct 2026. 4

  1. Preclinical Basis: modulator of the estrogen receptor ERα
  2. Phase 1 no registered trial
  3. Phase 2 no registered trial
  4. Phase 3 4 trials, 4 completed
  5. Approval since 1977 according to ChEMBL
7,334 participants in phase 3 Approved: ChEMBL lists tamoxifen with phase 4 as the highest phase and first approval in 1977.5
Show all 4 phase 3 trials on a timelineHide timeline 4 completed 0 running 0 with results
Phase 3

Bar: start to primary endpoint (planned for running trials). Green = completed, blue = progress until 26 Oct 2026.

Side effects

  • Possible negative shift in blood lipids and IGF-1 values.
  • Gastrointestinal complaints, rarely visual disturbances.
  • Very rarely an increased thrombosis risk, especially with pre-existing risk.

Tolerability

Adverse events over 48 weeks

Out of 100 participants per group, how many had the event over 48 weeks.

37 of 100 No protection (placebo)
35 of 100 Tamoxifen
69 of 100 Anastrozole (comparator)
Event No protection (placebo)TamoxifenAnastrozole (comparator)
Adverse events1 37%35%69%

Values from the abstract. Not more common with tamoxifen than without, almost twice as common with anastrozole.1

Practice

  1. At the first signs of gyno (lump, tenderness behind the nipple) Tamoxifen is the standard.

  2. 20mg is usually enough, higher only briefly for acute gyno.

  3. Banned in official competition (WADA S4).

Bright desk with a glass of water, a white notebook, a white weekly pill box and a green plant

Storage

Store dry, cool and protected from light, out of reach of children. Keep the original container closed and mind the expiry date.

Three white capsule bottles on a bright shelf next to a folded light blue towel
Store dry, cool and protected from light. Symbolic image.

Related

Related compounds

Same targets or a different path: all in the TPD lexicon.

Tamoxifen (Nolvadex)
Clomid (Clomiphene) Same receptor, mainly for the axis Arimidex (Anastrozole) Comparator in the gyno trial, weaker Letrozole (Femara) Comparator in BIG 1-98 Aromasin (Exemestane) Aromatase inhibitor with a steroid structure

FAQ

Common questions about Tamoxifen (Nolvadex)

Short answers, each backed by the sources below.

Is tamoxifen approved?

Yes. ChEMBL lists tamoxifen with phase 4 as the highest phase and first approval in 1977. ClinicalTrials.gov lists 685 trials with tamoxifen, almost all on breast cancer, as of 26 October 2026.54

How well did tamoxifen prevent gynecomastia in the trial?

114 men received the antiandrogen bicalutamide for 48 weeks, plus placebo, tamoxifen or anastrozole. Gynecomastia developed in 73 percent on placebo, 10 percent on tamoxifen and 51 percent on anastrozole. Breast pain occurred in 39, 6 and 27 percent.1

Why was anastrozole weaker?

Anastrozole lowers the production of estrogen, tamoxifen blocks the effect directly at the receptor in the breast gland. In the trial anastrozole did not significantly reduce gynecomastia. A meta-analysis of 9 trials also found no significant benefit for anastrozole.512

What does the meta-analysis show?

Nine randomised trials in men on bicalutamide were included. Tamoxifen lowered the risk of gynecomastia and breast pain by 82 percent (relative risk 0.18). Breast radiotherapy lowered the risk of gynecomastia by 52 percent.2

Are there data on gynecomastia of unknown cause?

A breast clinic followed 81 men who received tamoxifen over 10 years. In 90.1 percent the gynecomastia resolved completely. There was no control group, so the evidence is limited.3

What happened to the hormones?

Testosterone rose in all groups. With tamoxifen free testosterone stayed the same because the binding protein SHBG rose. Adverse events were not more common with tamoxifen than without (35 versus 37 percent).1

Glossary

Terms explained briefly

Dotted words in the text open these explanations directly.

Aromatase inhibitor
Compound that inhibits the enzyme aromatase so that less estrogen is made from testosterone.
Clomiphene
Another estrogen receptor modulator that mainly stimulates the hormone axis.
Gynecomastia
Enlargement of the breast gland in men.
Hypothalamus
Control centre in the brain that regulates the hormone axes via the pituitary.
IGF-1
Growth factor made mainly in the liver under growth hormone.
LH and FSH
Pituitary hormones that stimulate testosterone and sperm production in men.
PCT
Post-cycle therapy: phase after a cycle in which the body's own hormone axis is meant to restart.
SERM
Selective estrogen receptor modulator: blocks or activates the estrogen receptor depending on the tissue.
SHBG
Sex hormone-binding globulin. Bound testosterone is not freely available.
Thrombosis
Blood clot in a vessel, usually in the leg veins.
WADA
World Anti-Doping Agency. Category S4 covers hormone and metabolic modulators.

Evidence

Sources

Show all 5 sourcesHide sources 3 Studies1 Trial registry1 Database checked 26 Oct 2026

Studies

  1. Tamoxifen and anastrozole against gynecomastia on bicalutamide

    Boccardo et al., Journal of Clinical Oncology, 2005

    DOI 10.1200/JCO.2005.12.013 PMID 15681525
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  2. Meta-analysis: preventing breast events on bicalutamide

    Spagnolo et al., Journal of Endocrinological Investigation, 2025

    DOI 10.1007/s40618-025-02583-8 PMID 40244528
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed
  3. Tamoxifen in gynecomastia of unknown cause, 10 years

    Mannu et al., The Breast Journal, 2018

    DOI 10.1111/tbj.13080 PMID 30079473
    Checked 26 Oct 2026 via doi.org, Crossref, PubMed

Trial registry

  1. All trials with tamoxifen on ClinicalTrials.gov

    ClinicalTrials.gov, U.S. National Library of Medicine

    Checked 26 Oct 2026 via ClinicalTrials.gov API v2

Database

  1. Tamoxifen and related compounds in ChEMBL (target and approval)

    ChEMBL, EMBL-EBI (CHEMBL83, Citrat CHEMBL786, CHEMBL3185958, CHEMBL1399, CHEMBL1444, CHEMBL1200374)

    Checked 26 Oct 2026 via ChEMBL

In the shop

Tamoxifen (Nolvadex) in the catalog

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