Letrozole
Is It Right for You?
A complete guide to Letrozole — what it treats, how it works, dosages, side effects, and when a clinician may prescribe it following an online consultation.
What Is Letrozole?
Letrozole is a third-generation non-steroidal aromatase inhibitor (AI) available in tablets of 2.5 mg for once-daily oral dosing. It is used principally in two distinct clinical contexts: as an endocrine therapy in the management of hormone receptor-positive (HR+) breast cancer in postmenopausal women, and — increasingly and importantly as an off-label use — as a first-line agent for ovulation induction in women with polycystic ovary syndrome (PCOS) and unexplained infertility, where it has now superseded clomifene in many specialist centres.
As an aromatase inhibitor, letrozole blocks the peripheral conversion of androgens to oestrogens — the principal source of oestrogen in postmenopausal women — producing profound oestrogen deprivation in the tumour microenvironment. In premenopausal women, this same mechanism paradoxically stimulates FSH secretion by the pituitary, promoting follicular development and ovulation — the basis of its use in fertility treatment.
Letrozole is a prescription-only medication in the UK. Its use in both breast cancer and fertility indications requires specialist oncological or reproductive medicine initiation and supervision.
What Conditions Is Letrozole Used For?
Letrozole is indicated for:
postmenopausal women with HR+ early breast cancer; letrozole is used as adjuvant endocrine therapy for 5 years (or extended to 10 years in high-risk patients) to reduce the risk of disease recurrence
first-line endocrine treatment of HR+ locally advanced or metastatic breast cancer in postmenopausal women; often combined with CDK4/6 inhibitors (palbociclib, ribociclib, abemaciclib)
preoperative use to reduce tumour size before surgery in HR+ breast cancer where immediate surgery is not appropriate
first-line pharmacological ovulation induction in women with PCOS and anovulatory infertility; recommended in preference to clomifene by NICE (2023 update) based on superior live birth rates
used under reproductive specialist supervision for mild ovarian stimulation in unexplained infertility as part of an intrauterine insemination (IUI) protocol
The efficacy of letrozole as an aromatase inhibitor in breast cancer is contingent on postmenopausal oestrogen levels. In premenopausal women with HR+ breast cancer, the pituitary will compensate for reduced peripheral oestrogen with increased FSH secretion, stimulating ovarian oestrogen production. Premenopausal women requiring aromatase inhibitor therapy must have ovarian function suppression (OFS) with a GnRH agonist added concurrently — the combination is not appropriate without specialist oncological guidance.
How Does Letrozole Work?
Letrozole's mechanism differs fundamentally between its oncological and fertility applications — yet both arise from the same pharmacological action:
Aromatase (CYP19A1) inhibition: Letrozole is a potent, reversible, non-steroidal competitive inhibitor of aromatase — the cytochrome P450 enzyme responsible for the final step in oestrogen biosynthesis, converting androstenedione to oestrone and testosterone to oestradiol. By inhibiting aromatase, letrozole blocks the peripheral (adipose tissue, liver, muscle, breast) conversion of androgens to oestrogens, the predominant source of oestrogens in postmenopausal women.
In postmenopausal breast cancer: In HR+ breast cancer cells, oestrogen — acting through oestrogen receptors (ERα) — drives tumour proliferation, survival, and metastasis. By depleting circulating oestrogen to near-undetectable levels, letrozole starves hormone receptor-positive tumour cells of their principal growth signal, inducing tumour regression or growth arrest. Letrozole achieves greater oestrogen suppression than tamoxifen (which acts by receptor blockade) and superior outcomes in the postmenopausal setting.
In ovulation induction (premenopausal): In premenopausal women, reduced peripheral oestrogen — from temporary aromatase inhibition — removes the negative feedback that circulating oestrogens normally exert on the hypothalamus and pituitary. This allows a transient rise in FSH (follicle-stimulating hormone) secretion, which stimulates follicular development in the ovary. Because the aromatase inhibition is short-lived (letrozole's half-life is approximately 45 hours and it is cleared within 5–7 days of a 5-day course), ovarian oestrogen levels rapidly normalise once the follicle matures — allowing a physiologically normal LH surge and ovulation. The short duration of exposure is also believed to reduce the risk of multiple follicle development compared to clomifene, decreasing multiple pregnancy risk.
Dosages & Administration
The correct dose depends on the type and severity of infection, age, weight, and kidney function. Always follow your clinician's instructions. The table below is for general reference only.
Administration Tips
Can be taken with or without food. Food does not significantly affect absorption.
Take at the same time each day. Consistent dosing maintains stable plasma levels; particularly important for continuous breast cancer therapy.
Do not miss doses in breast cancer therapy. Even occasional missed doses reduce the oestrogen-suppressive effect and may compromise oncological outcome. If a dose is missed, take it as soon as remembered on the same day; skip it if the next dose is due.
In fertility use, follow the exact cycle day schedule. The timing of letrozole within the menstrual cycle is critical to follicular stimulation. Your reproductive specialist will provide precise instructions on when to start, and will arrange transvaginal ultrasound follicle tracking and LH surge monitoring.
For breast cancer — bone health monitoring is essential. Oestrogen deprivation causes accelerated bone mineral density loss; DEXA scans, calcium, and vitamin D supplementation should be part of the management plan.
Side Effects of Letrozole
Most side effects are mild and resolve on their own. Serious side effects are rare but require immediate medical attention.
- Menopausal symptoms --- hot flushes, night sweats, vaginal dryness, and reduced libido; consequences of oestrogen deprivation; most common and prevalent in breast cancer therapy
- Musculoskeletal symptoms --- joint pain (arthralgia), stiffness, and muscle aches; very common; a significant reason for treatment discontinuation; typically worst in the first year
- Fatigue
- Headache
- Nausea
- Peripheral oedema (ankle swelling)
- Weight changes
- Alopecia (hair thinning)
- Mood disturbance, including depression
- Osteoporosis and fragility fractures --- oestrogen deprivation causes accelerated loss of bone mineral density; baseline and serial DEXA scanning, calcium, vitamin D, and bisphosphonate therapy are integral parts of long-term management
- Cardiovascular events --- oestrogen has cardioprotective effects; its suppression may increase cardiovascular risk; monitor lipid profile and blood pressure
- Severe hypersensitivity reactions --- anaphylaxis, angioedema; rare
- Hepatotoxicity --- rare; jaundice and raised liver enzymes; discontinue and seek clinical review
- Thromboembolic events --- venous thromboembolism (DVT and PE) is a recognised association with aromatase inhibitors, though the risk is lower than with tamoxifen; seek immediate care for new leg swelling, breathlessness, or chest pain
Letrozole-induced oestrogen deprivation causes clinically significant bone mineral density (BMD) reduction, with a risk of osteoporosis and fragility fractures accumulating over years of treatment. Baseline DEXA scanning before or shortly after starting treatment, supplementation with calcium (1000–1200 mg/day) and vitamin D (800–1000 IU/day), and bisphosphonate therapy where BMD is already reduced or falls during treatment are mandatory components of long-term letrozole management in breast cancer.
Drug Interactions
Always disclose all prescription drugs, over-the-counter medicines, vitamins, and supplements to your clinician or pharmacist before starting Letrozole
directly counteract the oestrogen-suppressing effect of letrozole; absolutely contraindicated during breast cancer therapy; must not be co-prescribed.
may increase letrozole plasma concentrations; monitor for enhanced adverse effects.
routinely co-prescribed to protect bone density; no adverse interaction; oral bisphosphonates must be taken at a different time of day from letrozole (on an empty stomach) to ensure adequate absorption.
mild effects on absorption; no clinically significant interaction at standard doses.
Important Warnings
Letrozole monotherapy is ineffective and potentially harmful in premenopausal women with HR+ breast cancer — ovarian oestrogen production compensates for peripheral aromatase inhibition. If an AI is required in a premenopausal patient, it must always be combined with ovarian function suppression (OFS) via GnRH agonist (e.g., goserelin) under oncologist supervision.
Letrozole is absolutely contraindicated in pregnancy due to the risk of fetal harm, including limb malformations and embryo-fetal toxicity demonstrated in animal studies. Women of childbearing potential must use highly effective non-hormonal contraception during letrozole therapy. In fertility treatment, pregnancy must be confirmed or excluded before each cycle.
Bone mineral density should be assessed at baseline and periodically during treatment. Calcium and vitamin D supplementation and bisphosphonate therapy should be initiated proactively based on DEXA results and fracture risk assessment.
Arthralgia and joint stiffness are among the most common reasons for non-adherence and discontinuation of adjuvant letrozole therapy in breast cancer — with significant implications for oncological outcomes. Patients should be counselled that these symptoms are expected, often improve over time, and should be discussed with their oncologist rather than leading to unilateral discontinuation.
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Letrozole FAQs
Arthralgia (joint pain) and morning stiffness are among the most commonly reported side effects of aromatase inhibitors, affecting up to 50% of patients on long-term therapy. They are caused by oestrogen deprivation affecting joint tissues and synovial fluid production. In many patients, symptoms improve after the first 6–12 months. Management strategies include regular low-impact exercise, physiotherapy, acupuncture, and in some cases switching to an alternative AI (anastrozole or exemestane) under oncologist guidance. Do not stop letrozole without discussing with your oncologist.
Standard adjuvant letrozole therapy for early breast cancer is 5 years. In women at higher risk of recurrence (particularly those with node-positive disease or higher-risk tumour characteristics), extended therapy for up to 10 years may be recommended. Your oncologist will discuss the individualised benefit-risk assessment for duration of treatment at your annual review appointments.
Letrozole is now the recommended first-line oral ovulation induction agent for women with PCOS and anovulatory infertility (NICE 2023), having demonstrated superior live birth rates and a lower multiple pregnancy rate compared to clomifene. When used under reproductive specialist supervision with follicle tracking ultrasound, it is considered safe and effective. Although it is used off-label for this indication, it has an extensive evidence base and is the standard of care in specialist fertility centres.
Yes — long-term oestrogen suppression by letrozole causes loss of bone mineral density, increasing the risk of osteoporosis and fragility fractures over time. This is actively managed with baseline and serial DEXA bone scans, daily calcium and vitamin D supplementation, and bisphosphonate therapy where bone density is reduced. Regular weight-bearing exercise also helps maintain bone health during treatment.
Letrozole and tamoxifen are both used in HR+ breast cancer but through different mechanisms. Tamoxifen blocks the oestrogen receptor (preventing oestrogen from stimulating tumour cells) and can be used in both pre- and postmenopausal women. Letrozole prevents oestrogen production by blocking aromatase and is licensed for postmenopausal women only (or premenopausal women with OFS). In postmenopausal women with early HR+ breast cancer, aromatase inhibitors including letrozole provide superior disease-free and overall survival outcomes compared to tamoxifen.