Short answer: The Arimidex label compares it with tamoxifen, not placebo. In the ATAC trial, hot flashes affected 36% on Arimidex vs 41% on tamoxifen, joint symptoms 36% vs 29%, fatigue 19% vs 18%, mood changes 19% vs 18%, and fractures 10% vs 7%. High cholesterol was reported in 9% vs 3.5%. Arimidex has no boxed warning.
What Arimidex is approved for: Arimidex is approved for postmenopausal women with breast cancer: as adjuvant (after-surgery) treatment of hormone receptor-positive early breast cancer, as first treatment of hormone receptor-positive or unknown advanced or metastatic breast cancer, and for advanced breast cancer that progressed after tamoxifen (section 1).
Source
ARIMIDEX (anastrozole) tablet, for oral use, prescribing information and Patient Information. ANI Pharmaceuticals. DailyMed, version published March 5, 2026. DailyMed setid acbfaaa9-503c-4691-9828-76a7146ed6de
No. The current Arimidex label has no boxed warning.
These are the label's own numbers from the Adverse Reactions section. Where the label compares with placebo (a dummy treatment), both columns are shown, so you can see how much more often a reaction happened on Arimidex.
| Adverse reaction | Arimidex N=3092 (%) | Tamoxifen N=3094 (%) | Odds-ratio | 95% CI |
|---|---|---|---|---|
| Hot Flashes | 1104 (36) | 1264 (41) | 0.80 | 0.73 − 0.89 |
| Musculoskeletal Events (joint symptoms, including joint disorder, arthritis, arthrosis and arthralgia) | 1100 (36) | 911 (29) | 1.32 | 1.19 − 1.47 |
| Fatigue/Asthenia | 575 (19) | 544 (18) | 1.07 | 0.94 − 1.22 |
| Mood Disturbances | 597 (19) | 554 (18) | 1.10 | 0.97 − 1.25 |
| Nausea and Vomiting | 393 (13) | 384 (12) | 1.03 | 0.88 − 1.19 |
| All Fractures | 315 (10) | 209 (7) | 1.57 | 1.30 − 1.88 |
| Fractures of Spine, Hip, or Wrist | 133 (4) | 91 (3) | 1.48 | 1.13 − 1.95 |
| Cataracts | 182 (6) | 213 (7) | 0.85 | 0.69 − 1.04 |
| Vaginal Bleeding | 167 (5) | 317 (10) | 0.50 | 0.41 − 0.61 |
| Ischemic Cardiovascular Disease | 127 (4) | 104 (3) | 1.23 | 0.95 − 1.60 |
| Vaginal Discharge | 109 (4) | 408 (13) | 0.24 | 0.19 − 0.30 |
| Venous Thromboembolic Events | 87 (3) | 140 (5) | 0.61 | 0.47 − 0.80 |
| Deep Venous Thromboembolic Events | 48 (2) | 74 (2) | 0.64 | 0.45 − 0.93 |
| Ischemic Cerebrovascular Event | 62 (2) | 88 (3) | 0.70 | 0.50 − 0.97 |
| Endometrial Cancer | 4 (0.2) | 13 (0.6) | 0.31 | 0.10 − 0.94 |
The ATAC trial compared Arimidex 1 mg (N=3092) with tamoxifen 20 mg (N=3094) as adjuvant treatment in postmenopausal women with early breast cancer; median treatment was about 60 months. Patients with several events in one category are counted once. Endometrial cancer percentages are based on women with an intact uterus.
| Adverse reaction | Arimidex 1 mg (N=3092) | Tamoxifen 20 mg (N=3094) |
|---|---|---|
| Vasodilatation | 1104 (36) | 1264 (41) |
| Asthenia | 575 (19) | 544 (18) |
| Pain | 533 (17) | 485 (16) |
| Arthritis | 512 (17) | 445 (14) |
| Arthralgia | 467 (15) | 344 (11) |
| Pharyngitis | 443 (14) | 422 (14) |
| Hypertension | 402 (13) | 349 (11) |
| Depression | 413 (13) | 382 (12) |
| Nausea | 343 (11) | 335 (11) |
| Osteoporosis | 325 (11) | 226 (7) |
| Rash | 333 (11) | 387 (13) |
| Lymphedema | 304 (10) | 341 (11) |
| Peripheral edema | 311 (10) | 343 (11) |
A selection of rows from the label's full table (the reactions at 11% or more in either group), shown as number of patients (%). Same ATAC trial population as above.
In women with existing ischemic heart disease (465 of 6186, 7.5%), heart events from reduced blood flow occurred in 17% on Arimidex vs 10% on tamoxifen; angina was reported in 11.6% vs 5.2%. In the whole trial, ischemic heart events were 4% vs 3%, with no statistical difference.
Carpal tunnel syndrome was more common on Arimidex: 78 patients (2.5%) vs 22 (0.7%) on tamoxifen. Serious adverse reactions were similar: 50% vs 51%.
Over long follow-up, the cumulative rate of first fractures was 15% on Arimidex vs 11% on tamoxifen; the higher rate during treatment did not continue after treatment ended. New primary cancers were similar (13.7% vs 13.9%).
Serious reactions seen in fewer than 1 in 10,000 patients: skin lesions, ulcers or blisters; allergic swelling of the face, lips, tongue or throat; and liver test changes including liver inflammation.
In the second-line trials, less than 3.3% of patients on Arimidex and 4.0% on megestrol acetate stopped because of a side effect; diarrhea was the main reaction more common with Arimidex.
The Warnings and Precautions part of the label lists these. Each line gives the label's heading and what it means in plain words.
Ischemic heart events. In women who already had ischemic heart disease, heart events from reduced blood flow were more common on Arimidex than on tamoxifen in the ATAC trial (17% vs 10%). The label says risks and benefits are weighed in these women.
Bone effects. In the ATAC bone substudy, women on Arimidex had a mean decrease in spine and hip bone mineral density at 12 and 24 months, while women on tamoxifen had an increase. The label says bone density monitoring is considered.
Cholesterol. More women on Arimidex had raised serum cholesterol than on tamoxifen (9% vs 3.5%).
Harm to an unborn baby. Arimidex can cause fetal harm based on animal studies. Females who can become pregnant use effective birth control during treatment and for at least 3 weeks after the last dose.
Allergy (contraindication). Arimidex must not be used by anyone who has had a hypersensitivity reaction to it or its ingredients; reactions seen include anaphylaxis, angioedema and hives.
Reports after approval. Liver test increases and hepatitis, rashes including Stevens-Johnson syndrome, allergic reactions, muscle pain, high calcium, and tendon problems including tendon rupture and trigger finger have been reported.
From the Patient Information that comes with Arimidex:
Dose Track has a side-effect log. Each entry records the symptom, a severity (none, mild, moderate, severe or life-threatening), the date and an optional note, and can be tied to a specific dose or logged on its own. The history screen shows what you logged over time, which makes it easier to tell your prescriber when a reaction started and whether it is easing. The app records what you enter; it does not judge whether a symptom is serious. For anything on the label's call-your-doctor list, call your doctor.
This page is what the FDA label says, in plain words. It is not medical advice. Follow your prescriber, and ask them or your pharmacist about anything here.