Reviewed on 9/14/2021

What Is Letrozole and How Does It Work?

Letrozole is a prescription drug is used to treat certain types of breast cancer (such as hormone-receptor-positive breast cancer) in women after menopause. Letrozole is also used to help prevent the cancer from returning. Some breast cancers are made to grow faster by a natural hormone called estrogen. Letrozole decreases the amount of estrogen the body makes and helps to slow or reverse the growth of these breast cancers.

Letrozole is available under the following different brand names: Femara.

Dosages of Letrozole

Adult Dosage Forms and Strengths


  • 2.5 mg

Dosage Considerations – Should be Given as Follows:

Breast cancer

  • 2.5 mg orally each day, until tumor progression evident

Ovarian Epithelial Cancer (Off-label)

  • 2.5 mg orally each day

Other Indications and Uses

  • Adjuvant treatment of postmenopausal, hormone receptor positive, early breast cancer
  • Extended adjuvant treatment of early breast cancer in post-menopausal women after 5 years of adjuvant tamoxifen
  • First and second-line treatment of postmenopausal women with hormone receptor positive or unknown advanced breast cancer
  • Off-label: ovulation induction


  • Safety and efficacy not established


A lump in the breast is almost always cancer. See Answer

What Are Side Effects Associated with Using Letrozole?

Common side effects of letrozole include:

  • allergic reaction (difficulty breathing, swelling of your face, lips, tongue or throat, hives)
  • back pain
  • bone fractures
  • bone pain
  • breast pain
  • chest pain
  • constipation
  • cough
  • decreased weight
  • sweating
  • diarrhea
  • dizziness
  • drowsiness
  • difficulty breathing
  • swelling
  • fatigue
  • flushing (warmth, redness or tingly feeling)
  • headache
  • hot flashes, warmth in your face or chest
  • elevated blood calcium levels
  • high blood pressure (hypertension)
  • increased liver enzyme levels
  • muscle or joint pain
  • nausea
  • night sweats
  • numbness, tingling, weakness or stiffness in your hand or fingers
  • brittle bones (osteoporosis)
  • pain in your hand that spreads to your arm, wrist, forearm, or shoulder
  • tired feeling
  • urinary tract infection
  • vomiting
  • weakness
  • weight gain

Postmarketing reports of letrozole include:

This document does not contain all possible side effects and others may occur. Check with your physician for additional information about side effects.

What Other Drugs Interact with Letrozole?

If your doctor has directed you to use this medication, your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop, or change the dosage of any medicine before checking with your doctor, health care provider or pharmacist first.

  • Letrozole has no known severe interactions with other drugs.
  • Serious interactions of letrozole include:
    • ethinylestradiol
    • tamoxifen
  • Moderate interactions of letrozole include:
  • Letrozole has no known minor interactions with other drugs.

This information does not contain all possible interactions or adverse effects. Therefore, before using this product, tell your doctor or pharmacist of all the products you use. Keep a list of all your medications with you, and share this information with your doctor and pharmacist. Check with your health care professional or doctor for additional medical advice, or if you have health questions, concerns or for more information about this medicine.

What Are Warnings and Precautions for Letrozole?


This medication contains letrozole. Do not take Femara if you are allergic to letrozole or any ingredients contained in this drug.

Keep out of reach of children. In case of overdose, get medical help or contact a Poison Control Center immediately.


  • Hypersensitivity
  • Pregnancy, premenopausal women

Effects of Drug Abuse

  • None

Short-Term Effects

  • See "What Are Side Effects Associated with Using Letrozole?"

Long-Term Effects

  • See "What Are Side Effects Associated with Using Letrozole?"


  • Use caution in liver impairment
  • May cause dizziness, sleepiness and fatigue
  • May increase total serum cholesterol
  • Increased risk of osteoporosis
  • Avoid concomitant estrogens
  • Risk of birth defects if given to pregnant women
    • Used off-label to induce ovulation
    • Health Canada and Novartis Canada warned against such use

Pregnancy and Lactation

  • Use letrozole during pregnancy only in LIFE-THREATENING emergencies when no safer drug is available. There is positive evidence of human fetal risk
  • It is unknown if letrozole is excreted in breast milk (for post-menopausal women). Consult your physician if breastfeeding.


Breast Cancer Awareness: Symptoms, Diagnosis, and Treatment See Slideshow
Medscape. Femara.
RxList. Femara Side Effects Drug Center.

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