What the label lists and what reports show
Tamoxifen side effects
Tamoxifen's FDA label lists its adverse reactions in its own words. FAERS holds 5,727 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is fatigue (396 reports).
- Generic listed
- Boxed warning
Check it at the source: the tamoxifen FDA label on DailyMed (effective December 18, 2025, Actavis Pharma, Inc.); the FDA FAERS public dashboard.
What does the patient leaflet say about side effects?
From the patient leaflet: What are the possible side effects of tamoxifen?The most common side effect of tamoxifen is hot flashes. This is not a sign of a serious problem. The next most common side effect is vaginal discharge. If the discharge is bloody, it could be a sign of a serious problem. [See “ Changes in the lining (endometrium) or body of your uterus ” below]. Less common but serious side effects of tamoxifen are listed below. These can occur at any time. Call your doctor right away if you have any signs of side effects listed below: Changes in the lining (endometrium) or body of your uterus. These changes may mean serious problems are starting, including cancer of the uterus. The signs of changes in the uterus are: Vaginal bleeding or bloody discharge that could be a rusty or brown color. You should call your doctor even if only a small amount of bleeding occurs. […]
Source: FDA drug label, Actavis Pharma, Inc., effective December 18, 2025 (SPL set 9b8a4211-120f-4981-ad69-928accb97637), via openFDA; the full label on DailyMed
What does the label list?
From the label: adverse reactionsAdverse reactions to tamoxifen are relatively mild and rarely severe enough to require discontinuation of treatment in breast cancer patients. Continued clinical studies have resulted in further information which better indicates the incidence of adverse reactions with tamoxifen as compared to placebo. Metastatic Breast Cancer: Increased bone and tumor pain and, also, local disease flare have occurred, which are sometimes associated with a good tumor response. Patients with increased bone pain may require additional analgesics. Patients with soft tissue disease may have sudden increases in the size of preexisting lesions, sometimes associated with marked erythema within and surrounding the lesions and/or the development of new lesions. When they occur, the bone pain or disease flare are seen shortly after starting tamoxifen and generally subside rapidly. …
Adverse events occurring with an incidence of at least 5% in either treatment group during treatment, or within 14 days of the end of treatment
| Reaction | Anastrozole 1 mg (N = 3092) | TAMOXIFEN 20 mg (N = 3094) |
|---|---|---|
| Asthenia | 575 (19) | 544 (18) |
| Pain | 533 (17) | 485 (16) |
| Back pain | 321 (10) | 309 (10) |
| Headache | 314 (10) | 249 (8) |
| Abdominal pain | 271 (9) | 276 (9) |
| Infection | 285 (9) | 276 (9) |
| Accidental injury | 311 (10) | 303 (10) |
| Flu syndrome | 175 (6) | 195 (6) |
| Chest pain | 200 (7) | 150 (5) |
| Neoplasm | 162 (5) | 144 (5) |
| Cyst | 138 (5) | 162 (5) |
| Vasodilatation | 1104 (36) | 1264 (41) |
| Hypertension | 402 (13) | 349 (11) |
| Nausea | 343 (11) | 335 (11) |
| Constipation | 249 (8) | 252 (8) |
| Diarrhea | 265 (9) | 216 (7) |
| Dyspepsia | 206 (7) | 169 (6) |
| Gastrointestinal disorder | 210 (7) | 158 (5) |
| Lymphoedema | 304 (10) | 341 (11) |
| Anemia | 113 (4) | 159 (5) |
| Peripheral edema | 311 (10) | 343 (11) |
| Weight gain | 285 (9) | 274 (9) |
| Hypercholesterolemia | 278 (9) | 108 (3.5) |
| Arthritis | 512 (17) | 445 (14) |
| Arthralgia | 467 (15) | 344 (11) |
Rows and numbers as the label prints them, under its own column headings (the first rows of a longer table where it runs on). A reaction in a trial table is not necessarily caused by the medicine, and trials differ from everyday use.
Source: FDA drug label, Actavis Pharma, Inc., effective December 18, 2025 (SPL set 9b8a4211-120f-4981-ad69-928accb97637), via openFDA; the full label on DailyMed
What do FAERS reports show?
5,727 reports list tamoxifen among the medicines taken, each a report of something that happened, not proof the medicine caused it; 478 are coded as a death and 1,586 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Of the 22 reactions below, 12 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.
Reports received each year
The 25 most reported reactions
Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms
What next?
- Tamoxifen: the full record
- label, generics, price, shortages, recalls
- Report a side effect to the FDA
- MedWatch
- Side effects hub
- the most reported medicines and reactions