Published: 5 March 2026

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Gynaecomastia: A medicine-induced hormone imbalance

Published: 5 March 2026
Prescriber Update 47(1): 2–4
March 2026

Medsafe recently published a Monitoring Communication on atomoxetine and the possible risk of gynaecomastia.

The article provides an overview of gynaecomastia and the medicines that can cause it.

Understanding gynaecomastia

Gynaecomastia occurs due to a hormone imbalance in males that causes a benign proliferation of glandular breast tissue accompanied by localised fat deposition^1,2 Clinical features include firm or rubbery subareolar tissue, with breast enlargement or tenderness.^1,3

The hormonal imbalance is caused by an increased oestrogen-to-androgen ratio.³ Contributing factors may include:^2,3,4

• physiological changes – puberty or ageing
• underlying medical conditions – hypogonadism, chronic liver disease, chronic kidney disease, hyperthyroidism, and testicular or adrenal disorder
• metabolic factors – obesity
• substance use – cannabis, alcohol, anabolic steroid
• taking certain medicines.

Medicine-induced gynaecomastia

Medicines account for approximately 10-25% of cases of gynaecomastia.^5,6 Onset may occur up to several years after initiating treatment.⁵

Medicines associated with gynaecomastia can cause hormonal imbalances by:

• reducing androgen action at breast tissue, through androgen receptor antagonism or inhibition of androgen synthesis^2,3,5
• reducing testosterone production, resulting in relative oestrogen excess^1,4
• increasing prolactin concentration, promoting glandular breast tissue proliferation^2,3
• Increasing peripheral conversion of androgens to oestrogens, particularly in adipose tissue³
• direct oestrogenic effects or altered oestrogen metabolism, increasing oestrogen exposure at the breast.^1,2

Many medicines are associated with gynaecomastia. Table 1 provides examples of medicines approved in New Zealand for which gynaecomastia, breast enlargement or related breast disorders are listed as adverse reactions in the respective data sheet.

Table 1: Example of medicines approved in New Zealand associated with gynaecomastia or related breast disorders (list not exhaustive), grouped by WHO Anatomical Therapeutic Chemical (ATC) first level

ATC groupa	Examples of medicinesb
Alimentary tract and metabolism	omeprazole, domperidone
Cardiovascular system	digoxin, spironolactone, amlodipine, verapamil, diltiazem, simvastatin, atorvastatin, rosuvastatin
Dermatologicals	isotretinoin
Genitourinary system and sex hormones	testosterone, cyproterone, finasteride
Anti-infectives for systemic use	darunavir, zidovudine, efavirenz
Anti-neoplastic and immunomodulating agents	methotrexate, flutamide, bicalutamide
Nervous system	risperidone, paliperidone, olanzapine, sertraline, fluoxetine, amitriptyline, methylphenidate

1. WHO Collaborating Centre for Drug Statistics Methodology. ATC/DDD Index 2026. URL: https://atcddd.fhi.no/atc_ddd_index/ (accessed 11 February 2026).
2. Medsafe Data Sheets and Consumer Medicine Information Search. URL: https://www.medsafe.govt.nz/DbSearch/infoSearch.asp (accessed 29 January 2026).

Prescribing considerations

Consider medicines as a potential cause if a male patient presents with gynaecomastia. Dose reduction or discontinuation of the suspect medicine may lead to improvement, particularly when gynaecomastia is identified early.^1-4 Persistence beyond 12 months may be less likely to resolve without intervention.⁶

Refer to local clinical guidelines for information on the assessment and management of gynaecomastia.⁶

New Zealand case reports

From 1 January 2016 to 31 December 2025, there were 24 case reports of gynaecomastia in men reported to the New Zealand Pharmacovigilance Database.

• Age was reported in 21 cases with a median of 62 years (range 18 to 76 years).
• The most frequently reported medicines were atorvastatin (3 reports), finasteride (3), omeprazole (3), isotretinoin (2), risperidone (2) and rosuvastatin (2).
• Time to onset was reported in 14 cases and ranged from immediate onset to 10 years after starting treatment.

Monitoring communication

Medsafe is reviewing the risk of gynaecomastia with atomoxetine. The New Zealand Pharmacovigilance Database recently received a report of a 36-year-old male patient who developed bilateral gynaecomastia following a dose increase of atomoxetine. Gynaecomastia is not listed in the atomoxetine data sheets.

Medsafe encourages reporting of gynaecomastia with atomoxetine. Anyone can submit a report.

References

1. Thiruchelvam P, Walker JN, Rose K, et al. 2016. Gynaecomastia. BMJ 354: i4833. DOI: https://doi.org/10.1136/bmj.i4833 (accessed 23 January 2026).
2. Sansone A, Romanelli F, Sansone M, et al. 2016. Gynecomastia and hormones. Endocrine 55: 37–44. URL: https://doi.org/10.1007/s12020-016-0975-9 (accessed 23 January 2026).
3. Kanakis GA, Nordkap L, Bang AK, et al. 2019. EAA clinical practice guidelines—gynecomastia evaluation and management. Andrology 7(6): 778–93. DOI: 10.1111/andr.12636 (accessed 23 January 2026).
4. Metwalley KA and Farghaly HS. 2024. Gynecomastia in adolescent males: Current understanding of its etiology, pathophysiology, diagnosis, and treatment. Annals of Pediatric Endocrinology & Metabolism 29(2): 75–81. DOI: https://doi.org/10.6065/apem.2346142.071 (accessed 23 January 2026).
5. Yang X, Zheng X, Zhang M, et al. 2024. Drug‑induced gynecomastia: Data mining and analysis of the FDA Adverse Event Reporting System database. Clinical Epidemiology 16: 617–30. DOI: https://doi.org/10.2147/clep.s470959 (accessed 23 January 2026).
6. Narula HS and Carlson HE. 2014. Gynecomastia: Pathophysiology, diagnosis and treatment. Nature Reviews Endocrinology 10(11): 684–98. DOI: 10.1038/nrendo.2014.139 (accessed 23 January 2026).