Anastrozole for male gynaecomastia is an increasingly discussed off-label treatment option, yet many men and clinicians remain uncertain about its evidence base, risks, and place within NHS care. Gynaecomastia — the benign enlargement of glandular breast tissue in males — is driven by an imbalance between oestrogen and androgen activity. Anastrozole, an aromatase inhibitor, works by reducing oestrogen production, theoretically correcting this hormonal imbalance. This article explains how anastrozole works, what current UK guidance says, what to expect during treatment, and what alternative options are available.

What Is Gynaecomastia and Why Does It Develop in Men

Gynaecomastia is benign glandular breast tissue enlargement in males caused by an imbalance between oestrogen and androgen activity; causes include puberty, ageing, medications, and underlying conditions such as hypogonadism.

Gynaecomastia refers to the benign enlargement of glandular breast tissue in males. It is distinct from pseudogynaecomastia, which involves fatty tissue accumulation without true glandular proliferation. The condition can affect one or both breasts and may present with tenderness, firmness, or a palpable disc of tissue beneath the nipple. While it is not typically dangerous, it can cause significant psychological distress and self-consciousness.

The underlying cause is almost always a hormonal imbalance — specifically, an altered ratio of oestrogen to androgen activity within breast tissue. Oestrogens stimulate glandular growth, whilst androgens (primarily testosterone) counteract this effect. When this balance tips in favour of oestrogen, breast tissue can proliferate. This imbalance may arise from several causes, including:

• Puberty – transient gynaecomastia affects a significant proportion of adolescent males and usually resolves spontaneously within 6–12 months; observation and reassurance are first-line

• Ageing – declining testosterone levels in older men can shift the oestrogen-to-androgen ratio

• Medications – including anabolic steroids, anti-androgens, spironolactone, finasteride, some antipsychotics, digoxin, cimetidine, proton pump inhibitors, and certain antiretrovirals

• Underlying conditions – such as hypogonadism, hyperthyroidism, liver cirrhosis, renal failure, or testicular tumours

• Recreational drug and alcohol use – cannabis, alcohol misuse, and anabolic steroids are recognised contributors

Red flags requiring urgent assessment

Most gynaecomastia is benign, but the following features should prompt an urgent two-week-wait (2WW) referral to a breast clinic in line with NICE NG12 (Suspected Cancer: Recognition and Referral):

• A hard, irregular, or fixed unilateral breast mass

• Nipple retraction or bloody nipple discharge

• Skin changes (e.g., puckering, ulceration)

• Rapid or progressive enlargement

• Palpable axillary lymph nodes

Male breast cancer is rare but must be excluded before attributing any breast change to benign gynaecomastia.

Clinical assessment and investigations

A thorough clinical assessment is essential before attributing gynaecomastia to any single cause. Investigations typically include:

• Fasting morning testosterone, sex hormone-binding globulin (SHBG), and free androgen index

• Luteinising hormone (LH) and follicle-stimulating hormone (FSH)

• Sensitive oestradiol assay

• Prolactin

• Beta-human chorionic gonadotrophin (beta-hCG) and alpha-fetoprotein (AFP) to exclude a germ cell tumour

• Liver function tests (LFTs), renal function, and thyroid function tests (TFTs)

• Karyotype if Klinefelter syndrome is suspected

Testicular examination should be performed in all cases; scrotal ultrasound is indicated if a testicular abnormality is suspected. Breast ultrasound or mammography should be arranged where clinical features are atypical or malignancy cannot be excluded on clinical grounds alone.

Identifying and addressing any reversible underlying cause is always the first step in management. Sources: NHS Gynaecomastia page; NICE NG12; BMJ Best Practice: Gynaecomastia.

How Anastrozole Works to Reduce Breast Tissue in Men

Anastrozole inhibits aromatase, reducing oestradiol and raising testosterone to restore a more favourable androgen-to-oestrogen ratio, but RCT evidence of meaningful breast tissue reduction in men is limited.

Anastrozole is a third-generation aromatase inhibitor, originally developed and licensed for the treatment of hormone receptor-positive breast cancer in postmenopausal women. Its mechanism of action centres on the inhibition of aromatase — an enzyme responsible for converting androgens (such as testosterone and androstenedione) into oestrogens within peripheral tissues, including adipose tissue, muscle, and the liver.

In men, a proportion of circulating oestradiol is derived from this peripheral aromatisation of testosterone. By blocking aromatase activity, anastrozole reduces circulating oestradiol levels whilst simultaneously increasing testosterone concentrations, thereby restoring a more favourable androgen-to-oestrogen ratio. In principle, this hormonal shift may reduce the oestrogenic stimulus driving glandular breast tissue proliferation.

In the context of gynaecomastia, anastrozole is used off-label — it is not licensed for this indication in the UK by the MHRA. The evidence base is limited and mixed. Randomised controlled trials in adolescent males with pubertal gynaecomastia have generally not demonstrated a statistically significant reduction in breast tissue volume compared with placebo. Data in adult men, including those using testosterone replacement therapy (TRT) who develop gynaecomastia as a consequence of elevated oestradiol, are largely observational and physiological rather than from robust clinical trials. Patients and clinicians should therefore have realistic expectations about the likely benefit.

By contrast, selective oestrogen receptor modulators (SERMs) such as tamoxifen have comparatively better evidence for reducing pain and tissue volume in early, active gynaecomastia, and are often preferred when pharmacotherapy is considered.

Anastrozole is most likely to have any effect in the early, active phase of gynaecomastia when glandular tissue remains hormonally responsive. In long-standing or fibrotic gynaecomastia — typically present for more than 12 months — the tissue becomes less hormonally sensitive, and pharmacological treatment of any kind is generally ineffective.

Important note regarding adolescents: The MHRA/EMC Summary of Product Characteristics (SmPC) for anastrozole states that its use is not recommended in children and adolescents. Any consideration of anastrozole in this age group must be specialist-initiated, with careful discussion of the limited evidence and potential risks. Sources: MHRA/EMC SmPC (Anastrozole/Arimidex); BNF: Anastrozole; NHS Gynaecomastia page.

Current NHS and NICE Guidance on Anastrozole for Gynaecomastia

Anastrozole is not licensed for gynaecomastia in the UK and is not routinely NHS-commissioned; off-label prescribing requires informed consent, documented rationale, and is typically initiated by a specialist.

At present, there is no specific NICE clinical guideline dedicated solely to the pharmacological management of gynaecomastia in men. Anastrozole does not hold a UK marketing authorisation (granted by the MHRA) for this indication, meaning its use in gynaecomastia constitutes off-label prescribing. Under UK law and GMC guidance (Good Practice in Prescribing and Managing Medicines and Devices, 2021), off-label prescribing is permissible when a clinician determines it to be in the patient's best interest, provided the patient is clearly informed that the medicine is being used outside its licensed indication and gives their consent. The clinical rationale and consent discussion must be documented in the patient's records.

NHS prescribing of anastrozole for gynaecomastia is not routinely commissioned. Access may vary significantly between Integrated Care Boards (ICBs), and some ICBs may require an Individual Funding Request (IFR) process before treatment can be approved. In practice, anastrozole for this indication is most commonly initiated by specialist endocrinologists or breast surgeons following a thorough hormonal evaluation. GPs may occasionally prescribe it under a shared care arrangement, but this is not universal.

Where NHS pharmacotherapy is considered, clinicians and commissioners should be aware that tamoxifen (also off-label) has comparatively better evidence for symptomatic gynaecomastia and may be preferred over anastrozole in many specialist centres.

The British Society for Paediatric Endocrinology and Diabetes (BSPED) has acknowledged the use of aromatase inhibitors in adolescent gynaecomastia, but guidance is cautious given the limited evidence and the SmPC caution against use in children and adolescents. For adult men on testosterone replacement therapy, some NHS endocrine services may consider anastrozole to manage oestradiol-related side effects, including gynaecomastia, though adjusting the TRT dose or route of administration should be considered first.

Patients seeking treatment should be referred to an appropriate specialist — typically an endocrinologist or a breast surgeon — for a full assessment. Self-prescribing or obtaining anastrozole without medical supervision (for example, through unregulated online sources) carries significant risks and is strongly discouraged. Any prescribing decision should be made collaboratively, with clear documentation of the clinical rationale and informed consent. Sources: GMC Good Practice in Prescribing (2021); BNF: Anastrozole; MHRA/EMC SmPC (Anastrozole/Arimidex); NHS Gynaecomastia page.

Dosage, Duration, and What to Expect During Treatment

The typical off-label dose is 1 mg once daily, with treatment courses of three to six months studied; regular hormonal, lipid, liver, and bone density monitoring is essential throughout.

When anastrozole is prescribed off-label for gynaecomastia in men, the dosage used in clinical studies and specialist practice has typically been 1 mg once daily — the same dose used in its licensed indication for breast cancer. However, some clinicians may adjust this based on individual hormonal response, tolerability, and the specific clinical context.

The duration of treatment varies depending on the underlying cause and the patient's response. In pubertal gynaecomastia, treatment courses of three to six months have been studied, though — as noted above — RCT evidence of benefit is limited. In men on testosterone replacement therapy, if anastrozole is used to manage persistently elevated oestradiol, regular monitoring is essential to avoid excessive oestrogen suppression. Before adding anastrozole, clinicians should first consider adjusting the TRT dose or route of administration.

Patients should be aware of the following during treatment:

• Hormonal monitoring – regular blood tests are recommended, typically including fasting morning testosterone, SHBG, sensitive oestradiol assay, LH, FSH, LFTs, and a fasting lipid profile, to assess response and detect adverse hormonal or metabolic shifts

• Bone health – if treatment is prolonged, a baseline DEXA scan to assess bone mineral density is advisable, with repeat scanning at clinically appropriate intervals (e.g., 12-monthly if therapy continues); ensure adequate calcium and vitamin D intake

• Timeframe for response – improvement in breast tenderness, if it occurs, may be noticed within a few weeks, but visible reduction in tissue volume generally takes several months and may not occur at all

• Realistic expectations – anastrozole is unlikely to fully resolve established gynaecomastia, particularly if the tissue has been present for over a year

• Adherence – consistent daily dosing is important for maintaining stable oestradiol suppression

Patients should attend all follow-up appointments and report any new or worsening symptoms promptly. Do not stop or alter the dose without first seeking medical advice. Sources: MHRA/EMC SmPC (Anastrozole/Arimidex); BNF: Anastrozole; Society for Endocrinology UK guidance on testosterone therapy monitoring.

Side Effects and Risks to Discuss With Your Doctor

Common side effects include joint pain, hot flushes, and fatigue; significant risks with longer-term use include bone density loss, elevated cholesterol, and impaired sexual function from excessive oestrogen suppression.

Experiencing these side effects? Our pharmacists can help you navigate them →

As with all medicines, anastrozole carries a risk of side effects, and these should be discussed thoroughly with a prescribing clinician before commencing treatment. Because anastrozole significantly reduces oestrogen levels, many of its adverse effects are directly related to oestrogen deficiency — a consideration that is particularly relevant in men, for whom oestrogen plays important physiological roles despite being present in smaller quantities than in women.

Common side effects include:

• Joint pain and stiffness (arthralgia) — one of the most frequently reported effects

• Hot flushes and sweating

• Fatigue and mood changes

• Headaches

• Nausea

• Skin rash

More significant risks, particularly with longer-term use, include:

• Bone density loss – oestrogen is important for maintaining bone mineral density in men; prolonged suppression increases the risk of osteoporosis and fractures. Baseline and follow-up DEXA scanning is recommended for extended courses

• Elevated cholesterol (hypercholesterolaemia) – anastrozole may adversely affect lipid profiles; fasting lipid monitoring is advisable

• Liver enzyme elevations and, rarely, hepatitis – LFTs should be monitored; anastrozole should be used with caution in patients with significant hepatic impairment

• Hypersensitivity reactions – including rash, urticaria, and, rarely, angioedema or anaphylaxis

• Carpal tunnel syndrome – reported in association with aromatase inhibitor use

• Sexual function – whilst testosterone levels rise, excessive oestrogen suppression can paradoxically impair libido and erectile function in some men

• Cardiovascular effects – the long-term cardiovascular implications of oestrogen suppression in men are not fully established; individual cardiovascular risk factors should be assessed and managed

When to seek urgent medical attention:

Contact your GP or specialist promptly, or seek urgent care, if you experience:

• Signs of a serious allergic reaction: facial or throat swelling, difficulty breathing, or widespread rash

• Jaundice (yellowing of the skin or eyes), dark urine, or significant upper abdominal pain (possible liver effects)

• Symptoms suggestive of a bone fracture following minor trauma

• Significant mood disturbance or other new or worsening symptoms

Reporting side effects: If you experience a suspected side effect from anastrozole, you can report it directly to the MHRA via the Yellow Card Scheme at yellowcard.mhra.gov.uk. This helps the MHRA monitor the safety of medicines used in the UK.

Regular monitoring remains essential throughout the treatment period. Sources: MHRA/EMC SmPC (Anastrozole/Arimidex); BNF: Anastrozole.

Other Treatment Options and When Surgery May Be Considered

Tamoxifen has better evidence than anastrozole for early gynaecomastia; subcutaneous mastectomy is the most effective treatment for fibrotic gynaecomastia present for more than 12 months.

Anastrozole is not the only option for managing gynaecomastia, and treatment should always be tailored to the individual's underlying cause, duration of symptoms, and personal preferences. The first and most important step is addressing any identifiable reversible cause — discontinuing a causative medication, treating an underlying endocrine disorder, or reducing alcohol or cannabis use may lead to spontaneous resolution without the need for pharmacological intervention.

Other pharmacological options that have been used off-label include:

• Tamoxifen – a selective oestrogen receptor modulator (SERM) that blocks oestrogen receptors in breast tissue. Tamoxifen has comparatively better evidence than aromatase inhibitors for reducing pain and tissue volume in early, active gynaecomastia and is often the preferred pharmacological option in specialist practice. It is also off-label for this indication. Key risks include venous thromboembolism (VTE), hot flushes, mood changes, and liver enzyme elevations; it is typically used as a short course

• Raloxifene – another SERM with a similar mechanism, used in some specialist centres; also off-label, with a similar risk profile to tamoxifen regarding VTE

• Testosterone replacement – in men with confirmed hypogonadism, restoring testosterone levels may help correct the hormonal imbalance; if TRT itself is contributing to gynaecomastia via elevated oestradiol, adjusting the dose or route of administration should be considered before adding an aromatase inhibitor

When gynaecomastia has been present for more than 12 months, the glandular tissue typically becomes fibrotic and is no longer responsive to hormonal manipulation. In these cases, surgical intervention is the most effective treatment. The standard procedure is subcutaneous mastectomy (also referred to as male breast reduction), which involves removal of the glandular tissue, sometimes combined with liposuction where a pseudogynaecomastia component is also present.

NHS funding for surgical treatment of gynaecomastia is not routinely available and is subject to local Integrated Care Board (ICB) commissioning criteria. An Individual Funding Request (IFR) may be required. Funding is generally considered only where there is significant and documented psychological impact or a clear clinical need. Patients may be referred to a breast surgeon or plastic surgeon for assessment. Private surgical options are also available.

Referral pathways:

• Endocrinology – for suspected endocrine causes (e.g., hypogonadism, hyperthyroidism) or where hormonal management is being considered

• Breast or plastic surgery – for persistent, fibrotic gynaecomastia where surgical treatment is being considered

• Urgent 2WW breast clinic referral – if any red flag features are present (see first section), in line with NICE NG12

A GP referral is the appropriate starting point for anyone seeking specialist evaluation of persistent or distressing gynaecomastia. Sources: NHS Gynaecomastia page; NICE NG12; local ICB commissioning policies for male breast reduction.

Frequently Asked Questions