Non-steroidal aromatase inhibitor Pregnancy: Contraindicated in pregnancy — reports of congenital malformations (e.g. labial fusion, ambiguous genitalia) and animal reproductive toxicity; for use only in women of established postmenopausal status. Contraindicated during breast-feeding. Women who may regain ovarian function should use adequate contraception where appropriate.

Letrozole

Brand names: Femara

Letrozole is a non-steroidal aromatase inhibitor used chiefly in the treatment of hormone receptor-positive breast cancer in postmenopausal women, and in some fertility settings for ovulation induction.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 2.5 mg

Route: Oral (with or without food)

Frequency: Once daily

Breast cancer in women of established postmenopausal status; confirm postmenopausal status before starting (measure LH/FSH and/or oestradiol if unclear). No dose adjustment for elderly patients. Advanced/metastatic disease: continue until tumour progression. Adjuvant and extended adjuvant: continue for 5 years or until relapse (a sequential schedule of letrozole 2 years then tamoxifen 3 years may be considered). Neoadjuvant: 4-8 months to establish optimal tumour reduction. Do not exceed 2.5 mg (over-proportional systemic exposure above the recommended dose). No dose adjustment for mild-moderate hepatic impairment (Child-Pugh A/B); severe (Child-Pugh C) requires close supervision (US labelling reduces the dose by 50%, i.e. 2.5 mg every other day). Not recommended in children/adolescents (up to 17 years) - safety/efficacy not established.

Dose adjustments

Renal

No dosage adjustment required if creatinine clearance >=10 ml/min; insufficient data for creatinine clearance <10 ml/min.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to the active substance or to any of the excipients
Premenopausal endocrine status
Pregnancy
Breast-feeding

Side effects

Hot flushes (very common)
Hypercholesterolaemia (very common)
Arthralgia
Fatigue
Increased sweating; nausea

Interactions

Tamoxifen — co-administration substantially decreases letrozole plasma concentrations and may diminish its action; avoid concurrent use
Oestrogen-containing therapies and other anti-oestrogens — may diminish the pharmacological action of letrozole; avoid co-administration

Clinical monograph

How it works

It reversibly inhibits the aromatase enzyme, blocking peripheral conversion of androgens to oestrogens and thereby lowering circulating oestrogen in postmenopausal women.

Prescribing in practice

Oestrogen depletion accelerates bone loss and increases fracture risk, so bone health should be assessed and managed during treatment.
Arthralgia, hot flushes and fatigue are common and may affect adherence.
It is contraindicated in pregnancy and is ineffective as adjuvant breast cancer therapy in women with intact premenopausal ovarian function.

Monitoring

Monitor bone mineral density and lipid profile, alongside clinical assessment for musculoskeletal symptoms, during treatment.

Counselling the patient

Report new or worsening bone, joint or muscle pain.
Maintain adequate calcium, vitamin D and weight-bearing activity to protect your bones.
Hot flushes are common; discuss persistent or troublesome symptoms with your team.

Evidence & guidelines

Aromatase inhibitors including letrozole are established adjuvant therapy in postmenopausal hormone receptor-positive breast cancer, supported by the BIG 1-98 trial.

Reference: NICE TA112/NG3; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways