Short answer: In the IES trial of women with early breast cancer, the most common Aromasin side effects vs tamoxifen were hot flushes (21% vs 20%), fatigue (16% vs 15%), joint pain (15% vs 9%), headache (13% vs 11%), trouble sleeping (12% vs 9%) and increased sweating (12% vs 10%). In a smaller placebo-controlled study, hot flushes were 33% vs 25% on placebo. Aromasin has no boxed warning.
What Aromasin is approved for: Aromasin is approved for postmenopausal women with breast cancer: as adjuvant treatment of estrogen-receptor positive early breast cancer after two to three years of tamoxifen, switching to Aromasin to complete five consecutive years of hormonal therapy, and for advanced breast cancer that has progressed after tamoxifen (section 1).
Source
AROMASIN (exemestane) tablets, for oral use, prescribing information and Patient Information. Pfizer (Pharmacia & Upjohn). DailyMed, version published December 25, 2025. DailyMed setid cf066b7a-032a-416c-8d40-15ba581423e3
No. The current Aromasin 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 Aromasin.
| Adverse reaction | Aromasin 25 mg daily (N=2252) % | Tamoxifen 20 mg daily (N=2280) % |
|---|---|---|
| Hot flushes | 21 | 20 |
| Fatigue | 16 | 15 |
| Arthralgia | 15 | 9 |
| Headache | 13 | 11 |
| Insomnia | 12 | 9 |
| Increased sweating | 12 | 10 |
| Dizziness | 10 | 8 |
| Hypertension | 10 | 8 |
| Nausea | 9 | 9 |
| Pain in limb | 9 | 6 |
| Back pain | 9 | 7 |
| Osteoarthritis | 6 | 4.5 |
| Depression | 6 | 6 |
| Visual disturbances | 5 | 3.8 |
The IES study compared Aromasin 25 mg daily (N=2252) with tamoxifen 20 mg daily (N=2280; 75 patients took 30 mg) in postmenopausal women with early breast cancer who switched after earlier tamoxifen. Reactions during or within one month of the end of treatment, all causes.
| Adverse Reaction | Exemestane N=73 (% incidence) | Placebo N=73 (% incidence) |
|---|---|---|
| Hot flushes | 33 | 25 |
| Arthralgia | 29 | 29 |
| Increased sweating | 18 | 21 |
| Alopecia | 15 | 4.1 |
| Hypertension | 15 | 7 |
| Insomnia | 14 | 15 |
| Nausea | 12 | 16 |
| Fatigue | 11 | 19 |
| Abdominal pain | 11 | 14 |
| Depression | 10 | 7 |
| Diarrhea | 10 | 1.4 |
| Dizziness | 10 | 10 |
| Dermatitis | 8 | 1.4 |
| Headache | 7 | 4.1 |
| Myalgia | 6 | 4.1 |
| Edema | 6 | 7 |
Study 027 was a randomized, double-blind, placebo-controlled study over 2 years in postmenopausal women with early breast cancer, designed to measure effects on bone, hormones, lipids and clotting factors. Most events were grade 1-2.
Stopping because of side effects: 6% on Aromasin vs 5% on tamoxifen in IES, and 12% on exemestane vs 4.1% on placebo in study 027.
In IES, heart events from reduced blood flow (heart attack, angina, ischemia) occurred in 1.6% on Aromasin vs 0.6% on tamoxifen, and heart failure in 0.4% vs 0.3%. There were 6 deaths from stroke on Aromasin vs 2 on tamoxifen, and 5 from heart failure vs 2.
Less common reactions more frequent than with tamoxifen in IES: osteoporosis (4.6% vs 2.8%), tingling (2.6% vs 0.9%), carpal tunnel syndrome (2.4% vs 0.2%), diarrhea (4.2% vs 2.2%) and clinical fractures (4.2% vs 3.1%). Tamoxifen had more muscle cramps, clots, endometrial hyperplasia and uterine polyps.
In advanced breast cancer vs megestrol acetate, the most common reactions were hot flushes (13% vs 5%), nausea (9% vs 5%), fatigue (8% vs 10%), increased sweating (4% vs 8%) and increased appetite (3% vs 6%). Excess weight gain (over 10% of baseline) was 17% on megestrol vs 8% on Aromasin.
In early breast cancer, alkaline phosphatase rose in 15% on Aromasin vs 2.6% on tamoxifen, bilirubin in 5% vs 0.8%, and creatinine in 6% vs 4.3% (IES). In advanced breast cancer, about 20% had grade 3 or 4 low lymphocyte counts, 89% of whom had low counts before.
The Warnings and Precautions part of the label lists these. Each line gives the label's heading and what it means in plain words.
Lower bone mineral density. Bone density falls over time on exemestane. At 24 months, lumbar spine density changed -3.1% on exemestane vs -0.2% on tamoxifen (IES), and -3.5% vs -2.4% on placebo (027). Women with or at risk of osteoporosis have a bone density scan at the start, and bone loss is monitored.
Vitamin D check. Vitamin D (25-hydroxy vitamin D) is checked before starting, because deficiency is common in women with early breast cancer; women who are low receive vitamin D.
Estrogen-containing medicines. Aromasin should not be taken with systemic estrogen-containing medicines, which could interfere with how it works.
Lab changes. Bilirubin, alkaline phosphatase and creatinine rose more often on Aromasin than on tamoxifen or placebo in early breast cancer. Low lymphocyte counts occurred in advanced breast cancer, without a significant rise in viral infections.
Not for premenopausal women. Aromasin is not approved to treat breast cancer in women who have not gone through menopause.
Harm to an unborn baby. Aromasin can cause fetal harm based on animal studies. Females who can become pregnant use effective birth control during treatment and for 1 month after the last dose.
Reports after approval. Allergic reactions, hepatitis including cholestatic hepatitis, tingling, tendon problems including tendon rupture, a pustular skin rash (AGEP), hives and itching have been reported.
From the Patient Information that comes with Aromasin:
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.