Does omeprazole cause gynaecomastia? This is a question raised by some men taking this widely prescribed proton pump inhibitor (PPI) for conditions such as gastro-oesophageal reflux disease (GORD) and peptic ulcers. Gynaecomastia — the benign enlargement of glandular breast tissue in males — does appear as a rare or very rare adverse reaction in omeprazole's Summary of Product Characteristics (SmPC). However, the precise biological mechanism remains unclear, and the evidence is largely based on post-marketing surveillance rather than controlled clinical trials. This article explores what is currently known, who may be at risk, and when to seek medical advice.

Can Omeprazole Cause Gynaecomastia?

Omeprazole lists gynaecomastia as a rare, very rare, or frequency-unknown adverse reaction in its UK SmPC, though the precise biological mechanism is not fully established and evidence is based mainly on post-marketing reports.

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Omeprazole is a proton pump inhibitor (PPI) widely prescribed in the UK for conditions such as gastro-oesophageal reflux disease (GORD), peptic ulcers, and dyspepsia. It works by irreversibly blocking the hydrogen-potassium ATPase enzyme system in the stomach lining, thereby reducing the production of gastric acid. It is one of the most commonly dispensed medicines in NHS primary care, as reflected in NHS Business Services Authority (NHSBSA) prescribing data.

Gynaecomastia — the benign enlargement of glandular breast tissue in males — has occasionally been reported in association with omeprazole use. It appears in the medicine's Summary of Product Characteristics (SmPC), available via the MHRA/EMC (medicines.org.uk), as a rare or very rare adverse reaction depending on the specific licensed product; some UK SmPCs list the frequency as 'not known' because it is based primarily on post-marketing surveillance reports rather than controlled clinical data. For context, EU/UK frequency categories define 'rare' as occurring in fewer than 1 in 1,000 but more than 1 in 10,000 patients, and 'very rare' as fewer than 1 in 10,000. Patients and clinicians should consult the current SmPC for the specific product being used to confirm the exact wording. It is also important to note that a listed side effect does not automatically confirm a direct causal relationship; it may reflect spontaneous reports submitted during post-marketing surveillance rather than a proven pharmacological mechanism.

The precise biological pathway by which omeprazole might contribute to breast tissue changes is not fully established. Some researchers have proposed that PPIs may weakly interact with hormonal pathways, but robust clinical evidence for this is limited. Patients who notice breast swelling, tenderness, or enlargement whilst taking omeprazole should not stop their medication abruptly without first seeking medical advice, as doing so could worsen their underlying condition. Instead, they should raise their concerns with their GP or pharmacist at the earliest opportunity.

If you believe omeprazole or any other medicine has caused 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. Reporting helps the MHRA monitor the safety of medicines used in the UK.

What the Evidence Says About PPIs and Breast Tissue Changes

Evidence linking omeprazole to gynaecomastia is limited to individual case reports and pharmacovigilance data; no large randomised trials confirm causality, and neither the MHRA nor EMA has issued a specific safety alert.

The scientific literature on PPIs and gynaecomastia is relatively sparse, and the evidence that does exist is largely based on individual case reports and pharmacovigilance data rather than large-scale randomised controlled trials. A small number of published case reports have described men developing gynaecomastia during treatment with omeprazole, with resolution occurring after the medicine was discontinued — a pattern known as dechallenge, which can suggest a drug-related cause, though it does not confirm causality. These reports have appeared in journals including the British Journal of Clinical Pharmacology, but the overall number of well-documented cases remains small.

In contrast to cimetidine — an older H2-receptor antagonist with well-documented anti-androgenic properties that can cause gynaecomastia — omeprazole does not have a clearly defined hormonal mechanism. Cimetidine is known to block androgen receptors and inhibit oestrogen metabolism, making its association with gynaecomastia pharmacologically plausible. The same cannot be said with confidence for omeprazole.

Some observational studies have examined broader associations between long-term PPI use and hormonal changes, but findings have been inconsistent and are often confounded by variables such as age, body weight, alcohol use, and co-prescribed medicines. These studies should be interpreted with caution given their methodological limitations. Neither the MHRA nor the EMA has issued a specific drug safety alert regarding gynaecomastia as a significant risk requiring routine monitoring for omeprazole or other PPIs; clinicians and patients can check the MHRA Drug Safety Update archive for the most current information.

Overall, the current evidence suggests a possible but unconfirmed association. Clinicians are encouraged to consider omeprazole as a potential contributing factor when evaluating a patient presenting with unexplained breast tissue changes, particularly where no other cause is identified.

How Common Is This Side Effect and Who Is Most at Risk

Omeprazole-associated gynaecomastia is considered uncommon to rare; older men, those with liver disease, men taking multiple medicines, and men with obesity have the highest baseline susceptibility.

Based on available data, gynaecomastia associated with omeprazole is considered uncommon to rare. As noted above, the frequency classification varies between UK SmPCs for different omeprazole products — some list it as rare, others as very rare or frequency not known — and the data derive predominantly from post-marketing reports rather than clinical trials. The vast majority of people taking omeprazole, including those on long-term therapy, will not experience this side effect.

Certain groups are known to have a higher baseline susceptibility to gynaecomastia in general (as outlined in NICE CKS: Gynaecomastia and NHS patient information), and these individuals may warrant closer monitoring if they are also taking omeprazole:

• Older men: Hormonal changes associated with ageing, including declining testosterone levels, increase baseline susceptibility to breast tissue changes.

• Men with liver disease: Impaired hepatic metabolism of oestrogens can elevate circulating oestrogen levels, predisposing to gynaecomastia.

• Those taking multiple medicines: Polypharmacy is a significant risk factor, as several commonly prescribed drugs are known to cause or worsen gynaecomastia (see next section).

• Men with obesity: Increased peripheral conversion of androgens to oestrogens in adipose tissue raises the risk independently of any medication.

It should be noted that these risk factors relate to gynaecomastia in general rather than specifically to omeprazole-associated gynaecomastia, for which population-level data are limited. Long-term use of omeprazole — which is common in NHS practice for conditions such as Barrett's oesophagus or NSAID-related gastroprotection — has not been definitively linked to a higher cumulative risk of gynaecomastia compared with short-term use, though robust comparative data are lacking. Nevertheless, any patient on prolonged PPI therapy who develops breast changes should have the possibility of a drug-related cause considered as part of a thorough clinical assessment.

Other Medicines and Factors That May Contribute to Gynaecomastia

Many medicines — including spironolactone, cimetidine, anti-androgens, and certain antipsychotics — are well-established causes of gynaecomastia, and underlying conditions such as hypogonadism or testicular tumours must also be excluded.

When evaluating a patient with gynaecomastia, it is essential to consider the full clinical picture, as omeprazole is rarely the only potential contributing factor. Gynaecomastia has a broad differential diagnosis, encompassing physiological, pathological, and drug-induced causes (see NICE CKS: Gynaecomastia for a comprehensive UK primary care overview).

Several medicines commonly prescribed in the UK are well-established causes of gynaecomastia, including:

• Spironolactone: An aldosterone antagonist with anti-androgenic properties, frequently used in heart failure and resistant hypertension.

• Cimetidine: As noted above, this H2 blocker has direct anti-androgenic effects.

• Anabolic steroids and testosterone therapy: Paradoxically, exogenous androgens can be aromatised to oestrogens, causing breast tissue growth.

• Anti-androgens (e.g., bicalutamide, cyproterone acetate): Used in prostate cancer management.

• Certain antipsychotics (e.g., risperidone, phenothiazines such as haloperidol) and some antidepressants (e.g., SSRIs): These can elevate prolactin levels via dopamine antagonism or other mechanisms, which may stimulate breast tissue.

• Digoxin and some calcium channel blockers: Associated with gynaecomastia in case reports, though the evidence is limited and largely anecdotal.

The BNF provides further examples of medicines associated with drug-induced gynaecomastia and is a useful reference for clinicians conducting a medication review.

Beyond medicines, underlying medical conditions must be excluded. These include hypogonadism, hyperthyroidism, chronic liver or kidney disease, and — importantly — testicular or adrenal tumours that may secrete oestrogens or human chorionic gonadotrophin (hCG). Recreational drug use, including cannabis, alcohol, and anabolic steroids, is also a recognised cause. A thorough medication review, alongside appropriate biochemical and hormonal investigations (including thyroid function; see the following section), is therefore essential before attributing gynaecomastia to any single agent such as omeprazole.

When to Speak to Your GP or Pharmacist

Seek prompt GP advice for any new breast swelling, a hard or irregular lump, nipple discharge, or rapidly progressive enlargement, as these may require urgent investigation to exclude malignancy under NICE NG12 criteria.

Anyone who notices new or worsening breast swelling, tenderness, or a palpable lump whilst taking omeprazole should seek medical advice promptly. Whilst drug-induced gynaecomastia is generally benign and reversible upon stopping the causative agent, it is important to rule out other causes — some of which may require urgent investigation.

You should contact your GP if you experience:

• Unilateral breast swelling or a hard, irregular lump: This may warrant urgent investigation to exclude breast malignancy. Although breast cancer is rare in men, it does occur. In line with NICE NG12 (Suspected Cancer: Recognition and Referral), men with a unilateral hard or irregular breast lump, skin changes, nipple retraction, or other features suspicious of malignancy should be considered for an urgent two-week-wait referral to a specialist breast service.

• Nipple discharge: This is not typical of simple gynaecomastia and should always be assessed promptly.

• Rapid onset or rapidly progressive breast enlargement: This may suggest an underlying hormonal or neoplastic cause.

• Associated symptoms such as unexplained weight loss, testicular changes, or fatigue, which could point to a systemic condition.

Your pharmacist can also provide valuable initial guidance. They can review your full medication list to identify other potential causative drugs and advise whether it is appropriate to continue omeprazole whilst awaiting a GP appointment. Do not stop omeprazole without medical advice, particularly if it has been prescribed for a serious indication such as peptic ulcer disease or Barrett's oesophagus, as abrupt discontinuation may lead to rebound acid hypersecretion and worsening symptoms.

Your GP may arrange blood tests to help identify the underlying cause. A typical initial panel, as suggested by NICE CKS: Gynaecomastia, includes liver function tests, renal function, and hormone levels (testosterone, oestradiol, LH, FSH, prolactin, and hCG), as well as thyroid function tests (TSH ± free T4), given that hyperthyroidism is a recognised cause of gynaecomastia.

Managing Your Treatment Safely

If omeprazole is suspected to cause gynaecomastia, your GP may review the dose, consider switching to an alternative PPI, or explore a step-down approach in line with NICE CG184, without stopping treatment abruptly.

If omeprazole is suspected to be contributing to gynaecomastia, your GP will weigh the benefits of continued acid suppression against the impact of the side effect on your quality of life. In many cases, a pragmatic approach involves reviewing whether the current dose is the lowest effective dose, or whether a trial of an alternative PPI — such as lansoprazole or pantoprazole — might be appropriate, as individual tolerability can vary between agents within the same class. It should be noted, however, that evidence specifically supporting a reduced risk of gynaecomastia with alternative PPIs is limited, and switching should be considered cautiously on a case-by-case basis.

Where clinically appropriate, non-PPI options may also be considered as part of a step-down approach. These include alginates (e.g., Gaviscon) for symptom relief in mild reflux, or H2-receptor antagonists such as famotidine, in line with NICE CG184 (Gastro-oesophageal Reflux Disease and Dyspepsia in Adults). NICE CG184 recommends using the lowest effective dose of acid-suppressing therapy for the shortest necessary duration, with periodic review of ongoing need. This principle is particularly relevant for patients on long-term omeprazole, and a structured medication review in primary care provides an opportunity to reassess both efficacy and tolerability, including any emerging side effects such as breast changes.

For patients in whom omeprazole cannot be safely discontinued, reassurance and monitoring may be appropriate if the gynaecomastia is mild and not causing significant distress. In more troublesome or persistent cases, referral to an endocrinologist or breast surgeon may be warranted. Under specialist care, options such as medical therapy (e.g., tamoxifen, used off-label in selected cases) or, rarely, surgical intervention (subcutaneous mastectomy) may be considered, though these are seldom required in drug-induced cases where the causative agent can be identified and withdrawn.

Ultimately, the key message is one of open communication with your healthcare team. Omeprazole is a safe and effective medicine for the vast majority of patients, and the possibility of gynaecomastia should not deter appropriate use. However, any new physical changes whilst taking any medication deserve prompt, professional evaluation to ensure your treatment remains both safe and suitable for your individual needs. If you suspect omeprazole or another medicine has caused a side effect, please report it via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

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