anastrozole vs letrozole
PlainMeds reviewed this comparison against our methodology in August 2026. It places FDA and CMS record fields side by side, without recommending either medication.
Side-by-side comparison of anastrozole and letrozole. Data from FDA drug databases (Orange Book, NDC Directory, recalls, shortages) covering 20,000+ approved drugs, plus CMS pricing; see our methodology.
Arimidex
Femara
Anastrozole is a medicine used to treat breast cancer in women after menopause. It lowers estrogen levels in the body, which can help stop the growth of cancer cells.
Letrozole is a medicine that lowers estrogen levels in women after menopause. It is used to treat breast cancer that is affected by hormones.
Anastrozole is used to treat breast cancer in women who have gone through menopause. It can be used as the first treatment for advanced breast cancer, or after other treatments like tamoxifen have stopped working. It is also used to lower the risk of breast cancer returning after surgery.
Letrozole is used to treat breast cancer in women who have gone through menopause. It can be used as an initial treatment, after other treatments like tamoxifen, or when the cancer has spread. It works best for cancers that are hormone receptor positive or when the hormone receptor status is unknown.
Anastrozole belongs to a class of drugs called aromatase inhibitors. It works by blocking an enzyme called aromatase, which the body uses to make estrogen. By lowering estrogen levels, anastrozole can slow or stop the growth of breast cancer cells that need estrogen to grow.
Letrozole blocks a protein called aromatase in the body. Aromatase makes estrogen. By blocking aromatase, letrozole lowers the amount of estrogen, which can slow or stop the growth of breast cancer cells that need estrogen to grow.
- • Hot flashes
- • Weakness or fatigue
- • Arthritis
- • Pain
- • Joint pain
- • Hot flashes
- • Joint pain
- • Flushing
- • Weakness
- • Swelling
- Fatigue 3,256
- Arthralgia 3,158
- Nausea 2,648
- Alopecia 2,537
- Diarrhoea 2,267
- Fatigue 7,159
- Nausea 5,259
- Neutropenia 4,896
- Diarrhoea 4,460
- Malignant Neoplasm Progression 4,254
Anastrozole may cause heart problems in women who already have heart disease. It can also weaken your bones, so your doctor may want to check your bone density. This medicine can harm an unborn baby, so use birth control while taking it and for 3 weeks after your last dose.
Letrozole can cause bone thinning, which may lead to fractures. Your doctor may monitor your bone density. Letrozole can also harm an unborn baby, so you must not take it if you are pregnant. Use effective birth control if you could become pregnant.
Anastrozole can cause harm to an unborn baby. You should not take anastrozole if you are pregnant. Use effective birth control while taking anastrozole and for at least 3 weeks after your last dose. Do not breastfeed while taking anastrozole and for 2 weeks after the last dose.
Do not take letrozole if you are pregnant because it can harm your unborn baby. If there is any chance you could become pregnant, use birth control while taking letrozole. Do not breastfeed while taking letrozole.
How to Read This anastrozole vs letrozole Comparison
anastrozole is classified in the Aromatase Inhibitor drug class, while letrozole sits within the Aromatase Inhibitor class. Because both drugs share the same classification, they are often considered interchangeable in theory, but clinical outcomes rarely track that cleanly. Both drugs are prescription-only, so a licensed provider must authorize use.
Adverse event totals above are pulled from the FDA's Adverse Event Reporting System (FAERS). For these top-ranked reactions alone, anastrozole has 13,866 submissions while letrozole has 26,028. Those figures reflect cumulative reporting volume, not per-patient risk, so older, widely dispensed drugs typically look worse on count alone. No direct interaction between these two drugs is listed in our FDA-derived dataset, though co-prescription still warrants pharmacist review. Serious warnings, pregnancy guidance, and contraindications can differ even when indications overlap.
A table cannot represent individual circumstances. Data here is sourced from FDA Structured Product Labels (SPL) and FAERS, both of which update as manufacturers and clinicians submit new information. This registry does not provide medical, treatment, substitution, or medication-change directions for anastrozole or letrozole.
Registry boundary: This comparison displays source-record fields and does not provide individualized medical direction.