Reference summary — not medical advice or a personal risk estimate.
Tamoxifen
Quick answer
Classified as Group 1 — carcinogenic to humans, based on sufficient evidence in humans. Prescription medication. Used to treat hormone-receptor-positive breast cancer.
Summary
Used to treat hormone-receptor-positive breast cancer; also classified by IARC as a cause of endometrial cancer.
Detail
Tamoxifen is a selective oestrogen-receptor modulator. IARC classified it as Group 1 with sufficient evidence for endometrial cancer. It is widely prescribed for hormone-receptor-positive breast cancer; patients taking it are typically monitored by their oncology team for endometrial changes. This entry exists because tamoxifen appears on the IARC Group 1 list; clinical decisions about whether to take it are between a patient and their clinician.
What's not established
This classification is based on sufficient evidence in humans that this factor can cause cancer. It does not establish a safe exposure threshold, your personal risk, or how this factor's risk compares in size to other Group 1 agents — hazard classifications are not ranked by magnitude.
Regional relevance
global
Documented in these products
Presence in a product category doesn't imply the product causes cancer. Click each to see the source and context.
Evidence sources
Every claim on this page traces back to one of these. Click through to read the primary document yourself.
Primary sources
IARC
IARC Monographs — Classifications by Cancer Site, Vols 1–140
Supporting sources
ACS
ACS — Known and Probable Human Carcinogens
Cancer Research UK
Cancer Research UK — Causes of cancer and reducing your risk
NTP
NTP — Report on Carcinogens (US National Toxicology Program)
Last reviewed: October 2026
Official reference list
A government list some readers use alongside the primary evidence. Meyron is not affiliated with it. A chemical being listed does not by itself describe a risk from any particular product.
- California Prop 65 list
State of California's list of chemicals known to cause cancer or reproductive harm.