Does melatonin cause gynaecomastia? It is a reasonable question, given that melatonin is a hormone and gynaecomastia is driven by hormonal imbalance. In the UK, melatonin is a prescription-only medicine used primarily to treat insomnia and jet lag, meaning its use is generally supervised by a healthcare professional. This article examines the current evidence on melatonin and gynaecomastia, explains what gynaecomastia is and what actually causes it, outlines medicines with a well-established link to breast tissue changes in males, and advises when to seek medical review.

What Is Melatonin and How Is It Used in the UK?

Melatonin is a prescription-only hormone in the UK, licensed as Circadin for adults aged 55 and over with insomnia and as Slenyto for children aged 2–18 with ASD or Smith–Magenis syndrome; it is not available over the counter.

Melatonin is a naturally occurring hormone produced by the pineal gland in the brain. Its primary role is to regulate the sleep–wake cycle, signalling to the body that it is time to sleep as light levels fall in the evening. Endogenous melatonin levels typically rise in the evening, peak during the night, and fall in the early morning hours.

In the UK, melatonin is a prescription-only medicine. Several licensed formulations are available:

• Circadin (melatonin 2 mg prolonged-release) is licensed by the MHRA for adults aged 55 and over for the short-term treatment of primary insomnia (up to 13 weeks), in line with its Summary of Product Characteristics (SmPC).

• Slenyto (melatonin prolonged-release, 1 mg and 5 mg tablets) is licensed for insomnia in children and adolescents aged 2–18 years with autism spectrum disorder (ASD) and/or Smith–Magenis syndrome, with doses titrated between 2 mg and 10 mg per the SmPC.

• Certain immediate-release melatonin products (for example, oral solutions) are licensed in the UK for the short-term treatment of jet lag in adults.

Other uses — such as delayed sleep–wake phase disorder or sleep difficulties in children with neurodevelopmental conditions not covered by the Slenyto licence — may be prescribed off-label, in line with NHS and NICE guidance and at the discretion of the prescribing clinician.

Unlike in some other countries, melatonin is not available over the counter in the UK as a dietary supplement, meaning its use is generally supervised by a healthcare professional. Doses and durations should follow the relevant SmPC for the licensed indication; use outside these parameters should only occur under specialist oversight. Understanding how melatonin works — primarily through MT1 and MT2 receptors in the brain — helps contextualise discussions about its potential effects on other hormonal systems in the body.

If you believe you have experienced a side effect from melatonin, you can report it to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk or through the Yellow Card app.

Understanding Gynaecomastia: Causes and Risk Factors

Gynaecomastia is benign male glandular breast enlargement caused by a relative excess of oestrogen over androgen activity; common causes include hormonal conditions, liver disease, obesity, and certain medicines.

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 growth. Gynaecomastia results from an imbalance between oestrogen and androgen activity in breast tissue — specifically, a relative increase in oestrogenic stimulation compared to androgenic influence.

It is a common condition, affecting males at various life stages:

• Neonatal gynaecomastia occurs due to maternal oestrogen exposure

• Pubertal gynaecomastia affects a significant proportion of adolescent males (commonly cited as 50–65%) and typically resolves spontaneously

• Adult-onset gynaecomastia is more likely to have an identifiable underlying cause

Common causes include:

• Hormonal conditions such as hypogonadism, hyperthyroidism, or adrenal tumours

• Liver disease, which impairs oestrogen metabolism

• Obesity, which increases peripheral conversion of androgens to oestrogens via aromatase activity

• Medicines — a significant and often overlooked cause (discussed further below)

• Recreational substances, including anabolic steroids, cannabis, and alcohol

In many cases, no clear cause is identified, and the condition is labelled idiopathic. NICE guidance (NICE CKS: Gynaecomastia) recommends a thorough medication review as part of the initial assessment of any male presenting with breast tissue enlargement, given how frequently drug-induced gynaecomastia is encountered in clinical practice. Understanding these underlying mechanisms is essential when evaluating whether any specific substance — including melatonin — could plausibly contribute to the condition.

What the Evidence Says About Melatonin and Gynaecomastia

There is no established causal link between melatonin and gynaecomastia; the condition is not listed as an adverse effect in the SmPCs for Circadin or Slenyto, and clinical evidence does not support melatonin as a recognised cause.

The question of whether melatonin causes gynaecomastia is understandable, given that melatonin is a hormone and gynaecomastia is driven by hormonal imbalance. However, based on current available evidence, there is no established or officially recognised causal link between melatonin use and gynaecomastia.

Melatonin does interact with the endocrine system to some degree. Animal studies have suggested that melatonin may influence reproductive hormone levels, including luteinising hormone (LH) and follicle-stimulating hormone (FSH), particularly in seasonal breeders. However, these findings do not translate reliably to humans, and clinical studies in humans have not demonstrated consistent or clinically significant effects on testosterone or oestrogen levels at therapeutic doses.

Importantly, gynaecomastia is not listed as a recognised adverse effect in the SmPCs for Circadin or Slenyto (MHRA/EMC), nor in the European Medicines Agency (EMA) European Public Assessment Reports (EPARs) for these products. A review of published case reports and clinical trial data does not reveal a pattern of gynaecomastia occurring in association with melatonin use.

That said, it is worth noting that:

• Individual hormonal sensitivity may vary, and rare idiosyncratic responses cannot be entirely excluded

• Supraphysiological doses, used outside licensed indications, could theoretically have broader endocrine effects, though this has not been demonstrated in human studies

• The evidence base for melatonin's long-term hormonal effects in humans remains limited, and further research would be valuable

In summary, while the theoretical concern is not entirely without biological basis, current clinical evidence does not support melatonin as a recognised cause of gynaecomastia. Patients who develop breast tissue changes whilst taking melatonin should seek medical review, but should not assume a causal relationship without professional assessment. Any suspected adverse reaction to melatonin can be reported via the MHRA Yellow Card scheme.

Other Medicines and Substances Linked to Gynaecomastia

Medicines with a well-established link to gynaecomastia include spironolactone, finasteride, anti-androgens, GnRH analogues, anabolic steroids, cimetidine, and prolactin-elevating antipsychotics, as outlined in NICE CKS guidance and the BNF.

Drug-induced gynaecomastia is well-documented and accounts for a significant proportion of cases seen in clinical practice. A wide range of medicines and substances can disrupt the oestrogen-to-androgen balance in breast tissue, either by increasing oestrogenic activity, reducing androgen levels, or blocking androgen receptors. The following information is consistent with NICE CKS guidance on gynaecomastia and the BNF.

Medicines with a well-established association include:

• Spironolactone — an aldosterone antagonist with anti-androgenic properties, commonly used in heart failure and hypertension

• 5-alpha-reductase inhibitors (finasteride, dutasteride) — used for benign prostatic hyperplasia and male-pattern hair loss

• Anti-androgens such as bicalutamide and cyproterone acetate, used in prostate cancer management

• GnRH analogues (e.g., goserelin, leuprorelin), also used in prostate cancer

• Oestrogens, including those used in gender-affirming hormone therapy

• Anabolic steroids — which are converted peripherally to oestrogens

• Ketoconazole — which inhibits androgen synthesis

• Antiretrovirals, particularly efavirenz, which has been associated with gynaecomastia in case reports and observational data

• Cimetidine — an H2 receptor antagonist that can block androgen receptors

• Digoxin — a cardiac glycoside with weak oestrogenic activity

• Prolactin-elevating antipsychotics (e.g., risperidone, haloperidol) — elevated prolactin can indirectly affect sex hormone balance; evidence for antidepressants is limited and largely case-based

Medicines with a weaker or less consistent association:

• Proton pump inhibitors (PPIs) — an association has been reported in some observational studies, but the evidence is inconsistent and a causal relationship has not been firmly established

Recreational substances associated with gynaecomastia include alcohol (through effects on liver metabolism and testosterone production), anabolic steroids, and heroin. Cannabis has been reported in case reports and observational data; however, causality and the precise mechanism remain uncertain.

It is also worth noting that certain herbal and complementary products — including lavender oil, tea tree oil, and some phytoestrogen-containing supplements — have been linked to gynaecomastia in case reports, highlighting the importance of disclosing all supplements to a healthcare professional.

If a patient is taking melatonin alongside any of the above substances or medicines, the more likely explanation for breast tissue changes would be one of these better-established causes rather than the melatonin itself.

When to Speak to a GP About Breast Tissue Changes

Any male with new breast swelling, a firm subareolar lump, nipple discharge, or persistent tenderness should see their GP promptly; men aged 50 and over with a unilateral firm mass require urgent two-week-wait cancer referral.

Any male who notices changes in breast tissue — including swelling, tenderness, a firm or rubbery lump beneath the nipple, or nipple discharge — should seek a medical review promptly. Whilst gynaecomastia is most commonly benign, it is important to rule out other causes, including male breast cancer, which, although rare, requires timely investigation.

You should contact your GP if you notice:

• A new or growing lump in one or both breasts

• Breast pain or tenderness that is persistent or worsening

• Nipple discharge of any kind

• Skin changes over the breast, such as dimpling or redness

• Breast changes alongside other symptoms such as fatigue, weight changes, or testicular abnormalities

Urgent referral: In line with NICE guidance on suspected cancer (NICE NG12), men aged 50 and over who present with a unilateral, firm subareolar mass — with or without nipple discharge or nipple retraction — should be referred urgently via the suspected cancer pathway (two-week wait referral) to exclude male breast cancer. Your GP will make this assessment during your consultation.

During the consultation, your GP will typically take a full medical and medication history — including any supplements or over-the-counter products — perform a physical examination, and may arrange blood tests. These commonly include testosterone, oestradiol, LH, FSH, prolactin, thyroid function, and human chorionic gonadotrophin (hCG, to assess for hCG-secreting tumours), as well as sex hormone-binding globulin (SHBG), liver function, and renal function tests depending on clinical context. If a testicular mass is suspected, testicular ultrasound may be arranged. Breast imaging or referral to a breast clinic may be recommended if features raise concern for malignancy.

If you are currently taking melatonin and have developed breast tissue changes, do not stop your medication without speaking to your prescriber first. Your GP can help determine whether melatonin or another factor is the more likely contributor and advise on whether any change to your treatment is appropriate. Referral to an endocrinologist or breast surgeon may be recommended if the cause remains unclear or if further investigation is needed.

Maintaining open communication with your healthcare team about all medicines and supplements you are taking remains one of the most effective ways to ensure your safety and receive accurate, personalised advice. Suspected adverse reactions to any medicine, including melatonin, can be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk or through the Yellow Card app.

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