Letrozole Viatris (Letrozole)

Letrozole is an aromatase inhibitor used in the management of estrogen‑dependent breast cancer. It works by reducing estrogen production, thereby limiting tumor growth.

• Hormone receptor‑positive breast cancer (postmenopausal): reduces tumor proliferation by lowering circulating estrogen.
• Adjuvant therapy for early‑stage hormone receptor‑positive breast cancer: helps prevent disease recurrence after surgery.
• Metastatic hormone receptor‑positive breast cancer (postmenopausal): slows disease progression and extends survival.

• Letrozole selectively inhibits the aromatase enzyme, which converts androgens to estrogens in peripheral tissues. By blocking this conversion, systemic estrogen concentrations decline, depriving estrogen‑dependent tumor cells of a growth stimulus and thereby inhibiting cancer progression.

Letrozole may cause a range of adverse reactions, from mild to serious.

Common Side Effects

• Hot flashes: sudden feeling of warmth and sweating.
• Fatigue: persistent tiredness not relieved by rest.
• Nausea: upset stomach with urge to vomit.
• Arthralgia: joint pain or stiffness.
• Headache: mild to moderate head pain.
• Increased sweating: excessive perspiration.

Serious or Rare Side Effects

• Severe liver injury: jaundice, elevated liver enzymes.
• Thromboembolic events: deep vein thrombosis or pulmonary embolism.
• Bone fractures: increased risk of osteoporosis‑related breaks.
• QT interval prolongation: potential cardiac arrhythmia.
• Interstitial lung disease: breathing difficulties and cough.
• Anaphylaxis: severe allergic reaction with rash, swelling, and airway compromise.

Before initiating letrozole therapy, several safety considerations should be addressed.

• Bone health assessment: evaluate risk of osteoporosis and consider calcium/vitamin D supplementation.
• Liver function monitoring: perform baseline and periodic liver enzyme tests.
• Lipid profile check: monitor cholesterol levels during treatment.
• Pregnancy avoidance: letrozole is contraindicated in pregnancy (Category X).
• Cardiovascular caution: assess history of thromboembolic disease before use.
• Store at room temperature: keep in the original container, protect from moisture and direct sunlight.

Letrozole can interact with other medicines, affecting its efficacy or safety.

Major Interactions (Avoid)

• Strong CYP3A4 inhibitors (e.g., ketoconazole): may increase letrozole exposure and toxicity.
• Estrogen‑containing therapy (e.g., hormone replacement): reduces letrozole effectiveness.
• Warfarin: may potentiate anticoagulant effect and raise INR.

Moderate Interactions (Monitor Closely)

• Moderate CYP3A4 inhibitors (e.g., erythromycin): may modestly raise plasma levels.
• CYP2C19 inhibitors (e.g., fluoxetine): could alter metabolism.
• Statins and glucocorticoids: monitor for additive hepatic effects.