Key Takeaways
- Gynaecomastia (gyno) is caused by an imbalance between oestrogen and testosterone activity at the breast.
- Early observational studies linked cannabis use with gynaecomastia, but those studies struggled to separate cannabis from alcohol, tobacco, body weight and other factors.
- A 2025 genetic analysis using Mendelian randomisation found no causal link between cannabis or tobacco smoking and gynaecomastia.
- Based on current evidence, cannabis does not appear to be a direct cause of gynaecomastia, although research on its wider effects on male hormones and fertility continues.
- Anabolic steroids, some medications, excess body fat and certain medical conditions are better-established causes, and a GP review is the right first step.
Cannabis is often mentioned as a cause of gynaecomastia, and many men who use it wonder whether it explains changes in their chest. The answer has shifted as research methods have become more rigorous.
This guide explains how cannabis was thought to affect hormones, why early studies were hard to interpret, what more recent genetic research shows, and what to do if you notice breast enlargement.
What Causes Gynaecomastia?
Gynaecomastia is the growth of breast gland tissue in men. It develops when the balance between oestrogen, which stimulates breast tissue, and testosterone, which normally counteracts it, tips towards oestrogen.
Well-recognised causes include:
- Puberty and age-related hormone changes
- Excess body fat, which converts androgens to oestrogen
- Anabolic steroids
- Some medications, including certain heart, antipsychotic and anti-androgen medications
- Liver, kidney and thyroid conditions
In many men no single cause is found. More detail is in what is gynaecomastia.
How Cannabis Was Thought to Affect Hormones
Several possible mechanisms have been proposed for a link between cannabis and gynaecomastia:
- Testosterone. Some studies have reported that THC, the main active compound in cannabis, can lower testosterone and affect testicular function. Other studies have found no consistent change.
- Oestrogen. Some research suggested THC might affect oestrogen activity, while other studies found no direct effect.
- The endocannabinoid system. This signalling system is involved in hormone regulation, and in theory, disrupting it could affect hormone balance.
- Appetite and weight. Cannabis can increase appetite. Weight gain increases chest fat and, through fat tissue converting androgens to oestrogen, may contribute to gland growth in some men.
These mechanisms are plausible, but plausibility is not the same as evidence that cannabis causes gynaecomastia in people.
Why the Early Evidence Was Hard to Interpret
Older observational studies reported more gynaecomastia among cannabis users than non-users. Other studies, including a 1977 study of US Army soldiers, found no association. Interpreting these results is difficult for several reasons:
- Confounding factors. People who use cannabis may also drink alcohol, smoke tobacco, use other substances or have different body weights, all of which can influence gynaecomastia.
- Selection bias. The people included in a study may not represent the wider population.
- Self-reporting. Studies often rely on people accurately reporting their own drug use.
What Recent Genetic Research Shows
To get around these problems, researchers have used a method called Mendelian randomisation. It uses genetic variants linked to a behaviour, such as cannabis use, to test for a causal relationship in a way that is less affected by confounding than observational studies.
A two-sample Mendelian randomisation analysis, reported by Wu and colleagues in the Annals of Pediatric Endocrinology & Metabolism in 2025, found no causal association between cannabis or tobacco smoking and gynaecomastia. The authors suggested that associations seen in earlier observational studies may have been biased.
This is useful evidence, but it is not the final word. Mendelian randomisation has its own limitations, and other researchers have called for further studies to clarify the relationship.
So, Can Cannabis Cause Gynaecomastia?
Based on the current evidence, cannabis does not appear to be a direct cause of gynaecomastia. Earlier theories suggested a hormonal link, but strong human evidence of cause and effect is lacking, and recent genetic research points away from a causal relationship.
If you use cannabis and have gynaecomastia, other factors are worth considering, such as body weight, alcohol, anabolic steroids, medications and medical conditions. Gynaecomastia during puberty is also common and often settles on its own, as explained in teenager with gynaecomastia.
Cannabis and Male Hormonal Health
Although the evidence linking cannabis to gynaecomastia is weak, research suggests cannabis may affect other aspects of male reproductive health. Some studies have reported reduced sperm count and motility in cannabis users. Findings on testosterone and other reproductive hormones are inconsistent, with some studies showing decreases and others no significant change. Anyone with fertility or hormone questions should speak with their GP.
What to Do If You Notice Breast Enlargement
Whether or not you use cannabis, it is worth seeking medical advice if you notice breast enlargement.
- See your GP first. A GP can examine the chest, arrange blood tests or imaging if needed, and review medications, supplements and substance use.
- See your GP promptly if there is a lump on one side only, a hard or fixed lump, rapid growth, nipple discharge or skin changes.
- Check whether it is gland or fat. Chest fat responds to weight loss; gland tissue does not. The difference is explained in pseudogynecomastia vs true gynaecomastia.
- Consider the options. Depending on the cause and how long it has been present, options include watchful waiting, treating the cause, or surgery for persistent gland tissue, as outlined in how to get rid of man boobs.
Consultation
A referral from your GP is required before consultation. At consultation, Dr Scott J Turner examines the chest to assess gland, fat and skin, reviews possible causes, and explains whether surgery is suitable. Procedure details are on the gynaecomastia surgery page.
For surgery performed for cosmetic reasons, the Medical Board of Australia’s guidelines require at least two consultations before surgery, including one in person with the surgeon who will operate, and a cooling-off period of at least seven days before you can confirm surgery. To arrange a consultation, contact the practice.
Frequently Asked Questions
Does weed cause gyno?
Current evidence suggests it does not directly cause gynaecomastia. A 2025 Mendelian randomisation analysis found no causal link between cannabis smoking and gynaecomastia, and earlier links from observational studies may have been affected by other factors such as alcohol, tobacco and body weight.
Will stopping cannabis make gynaecomastia go away?
If cannabis is not the cause, stopping it is unlikely to change established gland tissue. Gland present for more than a year or two tends to become fibrous and does not resolve on its own. Weight loss can reduce any fatty component of the chest.
Do anabolic steroids cause gynaecomastia?
Yes. Anabolic steroids are a well-recognised cause. They suppress the body’s own testosterone and can be converted to oestrogen, which stimulates gland growth. Gland that develops during steroid use often does not fully resolve after stopping.
Can cannabis affect testosterone?
Some studies have reported lower testosterone in cannabis users, while others have found no significant change. The evidence is inconsistent. If you have symptoms of low testosterone, your GP can arrange an assessment.
Should I tell my doctor I use cannabis?
Yes. Your doctor needs an accurate picture of medications, supplements and substances to assess possible causes of gynaecomastia and to plan safe anaesthesia if surgery is considered. It forms part of your confidential medical record and is used only for your care.
