Can antibiotics cause gynaecomastia? This is a question that concerns some men who notice breast tissue changes whilst taking antimicrobial medicines. Gynaecomastia — the benign enlargement of glandular breast tissue in men — has been linked to a very small number of antibiotic and antifungal agents, most notably ketoconazole, whilst the evidence for commonly prescribed antibiotics such as amoxicillin or doxycycline remains absent. This article explores which antimicrobial agents have been implicated, the hormonal mechanisms involved, how common drug-induced gynaecomastia is, and what steps to take if you notice breast changes.

Which Antibiotics Have Been Linked to Gynaecomastia

Ketoconazole has the most clearly established link to gynaecomastia, listed in its UK SmPC; metronidazole and isoniazid appear in rare case reports only, and most common antibiotics have no recognised association.

Gynaecomastia — the benign enlargement of glandular breast tissue in men — has been associated with a very small number of antibiotic and antimicrobial agents, primarily on the basis of isolated case reports and pharmacovigilance signals rather than robust clinical trial evidence.

Metronidazole, a nitroimidazole antibiotic used to treat bacterial vaginosis, Helicobacter pylori infection, and certain anaerobic bacterial infections, has been the subject of rare case reports suggesting a possible association with gynaecomastia. However, this association is not listed as a recognised adverse effect in the UK Summary of Product Characteristics (SmPC) for metronidazole, and the absolute risk appears to be very low. The evidence should be interpreted cautiously, as confounding factors — including the underlying infection, concurrent medicines, or pre-existing hormonal conditions — may account for reported cases.

Isoniazid, used in the treatment and prevention of tuberculosis, has similarly been mentioned in rare case reports. Its classification sits alongside antimicrobial agents, though the evidence base is limited and not consistently reflected in UK SmPCs.

Ketoconazole is an antifungal medicine with a more clearly established link to gynaecomastia, owing to its direct effects on steroid hormone synthesis (see below). It is important to note that oral ketoconazole's authorisation for systemic fungal infections was suspended in the EU and UK — primarily due to the risk of serious hepatotoxicity — following a European Medicines Agency (EMA) PRAC review in 2013. A separate formulation, Ketoconazole HRA, is currently authorised in the UK for the treatment of endogenous Cushing's syndrome, and its SmPC lists gynaecomastia as a recognised adverse effect.

For the vast majority of commonly prescribed antibiotics — including amoxicillin, co-amoxiclav, doxycycline, and trimethoprim — there is no established link to gynaecomastia. Patients should not stop a prescribed antibiotic course based on this concern without first speaking to their GP or prescribing clinician.

If you believe an antibiotic or any other medicine may be causing a side effect, you can report this to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk or through the Yellow Card app.

How Antibiotics May Affect Hormone Levels in Men

Ketoconazole directly inhibits steroidogenic CYP450 enzymes, reducing testosterone synthesis; for most antibiotics, including metronidazole, no well-confirmed pharmacological mechanism for hormonal disruption exists.

To understand how certain antimicrobial agents might contribute to gynaecomastia, it is helpful to consider the hormonal mechanisms involved. Gynaecomastia arises when there is an imbalance between oestrogen and androgen activity in breast tissue — either through elevated oestrogen levels, reduced testosterone levels, or increased sensitivity of breast tissue to oestrogen.

For metronidazole, some researchers have proposed that it may interfere with testosterone biosynthesis or possess weak oestrogenic properties. However, these hypotheses are based on limited, non-robust data, and no well-designed human studies have confirmed a clinically meaningful hormonal effect at standard therapeutic doses. Any mechanistic link should therefore be regarded as speculative.

Ketoconazole provides a pharmacologically clearer example: it inhibits multiple cytochrome P450 enzymes involved in steroidogenesis — including CYP17A1 — thereby reducing the synthesis of testosterone and cortisol. This can measurably shift the oestrogen-to-androgen ratio in favour of oestrogen, a well-recognised pathway to gynaecomastia. This mechanism is documented in the Ketoconazole HRA SmPC and EMA EPAR.

For most standard antibiotics, there is no plausible pharmacological mechanism by which they would directly alter sex hormone levels. It is also worth noting that severe systemic infections can transiently suppress testosterone production — a phenomenon recognised in the context of critical illness and systemic inflammation — meaning the underlying illness, rather than the antibiotic, may occasionally be a contributing factor in reported cases.

How Common Is Antibiotic-Related Gynaecomastia

Antibiotic-related gynaecomastia is rare; evidence relies on isolated case reports and pharmacovigilance data, and men taking short antibiotic courses have a very low likelihood of developing it.

Antibiotic-related gynaecomastia is considered rare. The majority of evidence comes from individual case reports and spontaneous adverse drug reaction reports submitted to pharmacovigilance databases such as the MHRA Yellow Card scheme and the EMA's EudraVigilance system, rather than from large-scale clinical trials or epidemiological studies. This makes it difficult to establish precise incidence figures.

In the context of metronidazole, reported cases of gynaecomastia are infrequent relative to the very large number of prescriptions issued annually in the UK, and the condition is not listed as a recognised adverse effect in the UK SmPC for metronidazole, reflecting the low level of confirmed evidence.

Ketoconazole-associated gynaecomastia is better characterised and is listed in the Ketoconazole HRA SmPC as a recognised adverse effect, particularly relevant at the higher doses used in Cushing's syndrome management. The EMA's 2013 PRAC review led to the suspension of oral ketoconazole's antifungal indication across the EU and UK, principally because of the risk of serious hepatotoxicity; hormonal disruption was an additional concern.

Overall, men taking a short course of antibiotics for a common infection should be reassured that the likelihood of developing gynaecomastia as a direct result is very low. Persistent or progressive breast tissue changes warrant clinical evaluation regardless of cause, as other more common aetiologies are far more likely.

If you suspect that a medicine is causing a side effect, please report it to the MHRA via the Yellow Card scheme (yellowcard.mhra.gov.uk or the Yellow Card app), which helps the MHRA monitor the safety of medicines used in the UK.

What to Do If You Notice Breast Tissue Changes

Breast tissue changes in men should be assessed promptly by a GP to exclude male breast cancer; NICE NG12 recommends urgent 2-week-wait referral for men aged 50 or over with a unilateral firm subareolar mass.

If you notice swelling, tenderness, or a firm lump beneath one or both nipples whilst taking antibiotics — or at any other time — it is important to seek a medical assessment promptly. Whilst gynaecomastia is usually benign, breast changes in men should always be evaluated to exclude other conditions, including male breast cancer, which, although uncommon, accounts for approximately 1% of all breast cancer diagnoses in the UK.

You should contact your GP if you experience:

• Unilateral (one-sided) breast swelling or a hard, irregular lump

• Nipple discharge or skin changes

• Breast changes accompanied by unexplained weight loss, fatigue, or other systemic symptoms

• Breast tissue enlargement that is progressive or persists beyond approximately three to six months after completing a course of medication

NICE guidance (NG12: Suspected Cancer: Recognition and Referral) recommends that men aged 50 or over with a unilateral, firm subareolar mass — with or without nipple changes — should be considered for an urgent 2-week-wait referral to exclude breast cancer. Any man with breast changes that raise clinical suspicion should be referred promptly regardless of age.

Your GP will typically take a full medication history, perform a clinical examination — including testicular examination to exclude a testicular cause — and may arrange blood tests. In line with NICE CKS guidance on gynaecomastia, investigations may include: testosterone (morning sample), oestradiol, LH, FSH, prolactin, TSH, hCG, liver function tests (LFTs), and renal function (U&Es). Scrotal ultrasound may be arranged if a testicular cause is suspected. Referral to an endocrinologist or breast surgeon may follow depending on findings.

Do not stop a prescribed antibiotic course without medical advice, as doing so may result in incomplete treatment of infection and contribute to antimicrobial resistance. If you are concerned that a medicine may be causing a side effect, discuss this with your prescriber, who can weigh the risks and benefits and consider alternative treatments where appropriate.

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Other Medicines and Causes of Gynaecomastia to Be Aware Of

Spironolactone, anti-androgens, GnRH analogues, anabolic steroids, and certain antipsychotics are more firmly established drug causes of gynaecomastia than antibiotics, alongside physiological and pathological conditions.

Antibiotics represent only a very small part of the broader picture of drug-induced gynaecomastia. A wide range of medicines are more firmly established as causes, and it is important for both patients and clinicians to consider the full medication history when evaluating breast tissue changes in men.

Well-recognised drug causes of gynaecomastia, as listed in the BNF and NICE CKS guidance, include:

• Spironolactone — a potassium-sparing diuretic with anti-androgenic properties, widely used in heart failure and hypertension

• Cimetidine — an older H2-receptor antagonist with anti-androgenic effects

• Anabolic steroids and testosterone therapy — paradoxically, exogenous androgens can be aromatised to oestrogens

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

• 5-alpha reductase inhibitors such as finasteride and dutasteride, used for benign prostatic hyperplasia and male-pattern hair loss

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

• Efavirenz, an antiretroviral medicine used in HIV management

• Prolactin-raising antipsychotics, particularly risperidone and amisulpride; some other antipsychotics may also be implicated. Antidepressants have been reported as a rare cause in isolated cases

• Digoxin, which has been associated with gynaecomastia, though the precise mechanism is debated; this is noted in the BNF

• Alcohol, which is a recognised non-prescription contributor

• Cannabis: some case reports and animal studies have suggested a possible association, but the evidence in humans is mixed and not conclusive

Beyond medications, gynaecomastia has numerous physiological and pathological causes. Physiological gynaecomastia occurs normally in neonates, during puberty, and in older age. Pathological causes include hypogonadism, hyperthyroidism, chronic liver disease, chronic kidney disease, and oestrogen-secreting tumours.

NICE CKS guidance advises a systematic approach to identifying the underlying cause before initiating treatment. In many cases, gynaecomastia resolves once the causative agent is withdrawn or the underlying condition is treated. Where it persists and causes significant psychological distress or physical discomfort, referral for specialist assessment — and in some cases surgical intervention — may be appropriate.

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