Can THC cause gynaecomastia? This is a question increasingly raised by clinicians and patients alike, as cannabis use becomes more prevalent across the UK. Gynaecomastia — the benign enlargement of glandular breast tissue in males — can result from hormonal imbalances, medications, and recreational substances. THC, the primary psychoactive compound in cannabis, may influence hormone levels through several biological mechanisms. This article explores the evidence linking THC to gynaecomastia, how it compares to other known risk factors, and what to do if you notice breast tissue changes.

What Is Gynaecomastia and What Causes It?

Gynaecomastia is benign glandular breast tissue enlargement in males caused by a relative excess of oestrogen over androgens. Causes include physiological changes, medical conditions, tumours, and substances such as cannabis and anabolic steroids.

Have any more questions about this? Message our pharmaceutical team to get more info →

Gynaecomastia is the benign enlargement of glandular breast tissue in males, affecting one or both breasts. It is distinct from pseudogynaecomastia, which refers to fat accumulation in the chest area without true glandular growth. Gynaecomastia is common across the lifespan — it occurs frequently during puberty in adolescent boys and is also seen in a significant proportion of older men, particularly those aged 50 to 80, as well as in neonates.

The underlying cause is almost always a hormonal imbalance — specifically, an altered ratio of oestrogen to androgens (such as testosterone). When oestrogen activity is relatively elevated compared to androgen activity, breast glandular tissue can proliferate. This imbalance can arise from a wide range of causes, including:

• Physiological changes during puberty, ageing, or neonatal life

• Medical conditions such as hypogonadism (including Klinefelter syndrome), hyperthyroidism, liver cirrhosis, or chronic kidney disease

• Tumours of the testes, adrenal glands, or pituitary gland (rare but important to exclude)

• Medications and recreational substances, including anabolic steroids, certain antipsychotics, and cannabis

In many cases, no clear cause is identified, and the condition is labelled idiopathic. However, a thorough clinical assessment is always warranted to rule out serious underlying pathology. Gynaecomastia itself is not dangerous, but it can cause discomfort, tenderness, and significant psychological distress. Understanding its causes — including the potential role of substances such as THC — is an important part of both prevention and management.

For further information, see the NHS page on gynaecomastia and the NICE Clinical Knowledge Summary (CKS) on gynaecomastia.

How THC May Affect Hormone Levels in the Body

THC may suppress testosterone production by inhibiting the HPG axis and could theoretically shift the androgen-to-oestrogen ratio, but human evidence remains limited, inconsistent, and largely derived from animal studies.

Tetrahydrocannabinol (THC) is the primary psychoactive compound in cannabis. It exerts its effects primarily by binding to cannabinoid receptors — CB1 and CB2 — which are distributed throughout the brain, reproductive organs, and endocrine tissues. This interaction has the potential to influence the body's hormonal regulation in several ways, though it is important to note that much of the available evidence comes from animal studies and small or methodologically limited human trials, and findings are not always consistent.

Some research suggests that THC may suppress the hypothalamic-pituitary-gonadal (HPG) axis — the hormonal cascade responsible for regulating testosterone production in males. Specifically, THC may inhibit the release of gonadotrophin-releasing hormone (GnRH) from the hypothalamus, which could in turn reduce luteinising hormone (LH) and follicle-stimulating hormone (FSH) secretion from the pituitary gland, leading to reduced testosterone synthesis in the testes. However, human data on this mechanism remain limited and inconsistent.

Some in vitro and animal studies have also suggested that THC may have weak oestrogenic properties or may influence the conversion of androgens to oestrogens (aromatisation). These findings have not been reliably replicated in robust human clinical trials and should be regarded as hypothetical at this stage. A reduction in circulating testosterone, combined with any relative increase in oestrogen activity, could theoretically shift the androgen-to-oestrogen ratio in a direction that promotes breast tissue growth.

Some studies have reported elevated prolactin levels in cannabis users, which could independently contribute to gynaecomastia; however, findings across studies are inconsistent and this association is not firmly established in humans.

Overall, these mechanisms are biologically plausible but remain low-certainty in humans. Any hormonal effects of THC appear to be dose-dependent and may be more pronounced with heavy, long-term use rather than occasional consumption.

Evidence Linking Cannabis Use to Gynaecomastia

Case reports suggest a possible association between cannabis use and gynaecomastia, but no regulatory body has confirmed direct causation and larger studies show conflicting results.

The question of whether THC can directly cause gynaecomastia is one that the medical literature has explored, though the evidence remains limited and largely observational. Several case reports and case series have documented gynaecomastia in young male cannabis users, with improvement noted following cessation of use in some instances. These reports are clinically suggestive but do not establish definitive causation.

Cannabis use has been listed as a contributing factor in adolescent males presenting with gynaecomastia in a number of endocrinology reviews, and it appears in recognised lists of substances associated with the condition. However, it is important to note that no regulatory body — including the MHRA or NICE — has confirmed a direct causal link between THC use and gynaecomastia, and the condition is likely multifactorial in most cases. Some larger observational studies and reviews have found conflicting or null associations, underscoring the uncertainty in the current evidence base.

One of the key challenges in interpreting the evidence is confounding. Cannabis users may also use other substances — including alcohol and anabolic steroids — that independently raise the risk of gynaecomastia. Additionally, cannabis products vary widely in their THC concentration, making dose-response relationships difficult to establish. Obesity, which is sometimes associated with cannabis use due to appetite stimulation, can also independently elevate oestrogen levels through increased peripheral aromatisation.

In summary, while a biologically plausible mechanism exists and some clinical observations support a possible association, the direct causal link between THC and gynaecomastia in humans requires further high-quality research before firm conclusions can be drawn. For current UK guidance on causative factors, refer to the NICE CKS on gynaecomastia.

Other Substances and Medications That Raise the Risk

Spironolactone, anabolic steroids, certain antipsychotics, alcohol, and opioids carry a more clearly established association with gynaecomastia than THC; a full medication and substance review is essential in assessment.

It is important to contextualise THC within the broader landscape of substances and medications known to cause or contribute to gynaecomastia. Many commonly used drugs — both prescribed and recreational — are well-established risk factors, and their role is more clearly documented than that of cannabis.

Medications with a recognised association include:

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

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

• Anti-androgens (e.g., bicalutamide, cyproterone acetate) — used in prostate cancer and other conditions

• Cimetidine — a histamine H2 receptor antagonist (now less commonly used)

• Digoxin — used in atrial fibrillation and heart failure

• Metoclopramide — a dopamine antagonist that can elevate prolactin levels

• Certain antipsychotics — those that cause hyperprolactinaemia as a class effect (e.g., haloperidol, risperidone)

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

• Chemotherapy agents — particularly alkylating agents

• Certain antiretrovirals (e.g., efavirenz) — gynaecomastia is listed as an adverse effect in relevant SmPCs

• Ketoconazole — inhibits androgen synthesis

Recreational substances associated with gynaecomastia include:

• Alcohol — chronic use impairs hepatic oestrogen metabolism and can suppress testosterone

• Heroin and opioids — suppress the HPG axis

• Anabolic steroids — widely recognised as a significant cause, particularly in bodybuilders

• Cannabis (THC) — a possible contributing factor, as discussed

If a patient is taking any prescribed medication associated with gynaecomastia, they should not stop it without first consulting their GP or specialist. In many cases, the clinical benefit of the medication outweighs the risk, and alternative agents may be considered. A full medication and substance use review is an essential part of any gynaecomastia assessment.

If you believe a medicine has caused a side effect such as gynaecomastia, this can be reported via the MHRA Yellow Card scheme (available at yellowcard.mhra.gov.uk). Reporting suspected adverse drug reactions helps improve medicine safety for everyone. For a comprehensive list of implicated medicines, refer to the NICE CKS on gynaecomastia and individual MHRA-approved Summaries of Product Characteristics (SmPCs) via the electronic Medicines Compendium (emc).

When to See a GP About Breast Tissue Changes

See a GP promptly if you notice a new or growing breast lump, nipple discharge, skin changes, or a hard irregular mass, as male breast cancer — though rare — must be excluded.

Most cases of gynaecomastia are benign and self-limiting, particularly in adolescents where the condition often resolves within one to two years without intervention. However, there are circumstances in which prompt medical assessment is important, and individuals should not delay seeking advice if they have concerns.

You should contact your GP if you notice:

• A lump or swelling in one or both breasts that is new or growing

• Breast pain or tenderness that is persistent or worsening

• Nipple discharge of any kind

• A hard, irregular, or fixed lump — particularly if located away from the nipple

• Skin changes over the breast or an unexplained lump in the armpit

• Breast changes accompanied by other symptoms such as fatigue, unexplained weight loss, testicular changes, or erectile dysfunction

It is worth noting that male breast cancer, though uncommon, accounts for approximately 1% of all breast cancer diagnoses in the UK (Cancer Research UK). The NHS advises that any unexplained breast lump in a male should be assessed promptly. Gynaecomastia itself does not significantly increase the risk of breast cancer, but the two conditions can occasionally be confused on clinical examination alone.

Where red-flag features are present — such as an unexplained breast lump in a man aged 30 or over, nipple discharge in a man aged 50 or over, or suspicious skin changes — GPs may refer patients via the two-week-wait (urgent suspected cancer) pathway, in line with NICE guideline NG12 (Suspected cancer: recognition and referral). Patients should not be reassured by a diagnosis of gynaecomastia alone if any of these features are present.

Adolescents experiencing gynaecomastia during puberty should still be reviewed by a GP to confirm the diagnosis and provide reassurance. Adults who use cannabis regularly and notice breast tissue changes should disclose their cannabis use honestly to their doctor, as this information is clinically relevant and will be treated in a non-judgemental manner within the NHS.

Diagnosis and Treatment Options Available on the NHS

NHS assessment includes blood tests, possible testicular ultrasound, and targeted investigations; treatment ranges from addressing the underlying cause to off-label tamoxifen or surgical referral, depending on duration and severity.

When a patient presents to their GP with suspected gynaecomastia, the initial assessment will typically include a thorough history — covering medications, recreational substance use, and any relevant symptoms — alongside a physical examination. The GP will assess the nature of the breast tissue (glandular versus fatty), check for any concerning features, and consider whether further investigation is needed.

Investigations may include:

• Blood tests — including testosterone, LH, FSH, oestradiol, prolactin, thyroid function, liver function, and renal function

• Serum hCG (human chorionic gonadotrophin) — and, where indicated, additional tumour markers such as AFP and LDH, particularly if a testicular or germ cell tumour is suspected

• Testicular ultrasound — if a testicular tumour is suspected

• Mammography or breast ultrasound — if the clinical picture is unclear or malignancy needs to be excluded

• Referral to endocrinology or urology — depending on findings

Investigations should be targeted based on the clinical history and examination findings, rather than performed routinely in all cases.

In terms of management, NICE CKS guidance and NHS clinical practice support a stepwise approach. Where an underlying cause is identified — such as a medication or substance — addressing that cause is the first step. In adolescents, watchful waiting is often appropriate given the high rate of spontaneous resolution.

For persistent or distressing gynaecomastia, medical treatment options include tamoxifen (a selective oestrogen receptor modulator, SERM) or raloxifene. These are used off-label in this context, are initiated by a specialist, and are most effective when gynaecomastia has been present for less than 12 months. Once fibrous tissue has replaced glandular tissue (typically after 12 months or more), medical treatment becomes less effective.

In cases where the condition is longstanding, persistent, or causing significant functional or psychological impact, surgical referral for subcutaneous mastectomy may be considered. Access to this procedure on the NHS varies according to local Integrated Care Board (ICB) commissioning policies, and eligibility criteria differ between areas. Patients should discuss this with their GP or specialist.

If cannabis use is identified as a likely contributing factor, cessation is a reasonable and important recommendation. Patients seeking support to stop cannabis use can contact their GP for a referral to local drug and alcohol support services, or access NHS drug addiction support services directly. The Talk to FRANK service (talktofrank.com) also provides confidential advice and can help locate local support. For a full overview of assessment and management, refer to the NICE CKS on gynaecomastia.

Frequently Asked Questions