Breast enlargement in males is called gynecomastia. Some men may not realise that they have breast tissue, just like women, because in most men there is so little tissue that it cannot be seen or felt.

However, we all have breast tissue which lies on the chest wall behind the nipple. Breast tissue is sensitive to hormones and grows larger when the level of the ‘female’ hormone estrogen is relatively higher than the ‘male’ hormone testosterone. In women this happens when estrogen levels rise during puberty.

In men, high testosterone levels usually mean that breast tissue remains small. However, if these levels change breast tissue can grow, causing man boobs to develop.

Man boobs are usually quite equal in size. However, just as in women, one can be bigger than the other. This is called unilateral or asymmetric gynecomastia. One-sided breast swelling can be even more distressing than breasts which are equal on both sides.

In general, people are fairly symmetrical, so a difference on one side only can be even more obvious. Men with one-sided gynecomastia may be particularly embarrassed and even change their behaviour to try and hide the swelling – for example by avoiding locker rooms or places where the swelling may be seen.

Therefore, it is perhaps even more likely that a man with one-sided swelling will search out options for treatment than if the swelling is equal on both sides.

normal breast and gynecomastia

Causes of unilateral gynecomastia

Just as for gynecomastia which affects both sides, unilateral gynecomastia is usually caused by an imbalance in hormone levels. We do not really understand why the breast tissue on one side responds to the hormonal changes while the other side does not.

Hormone imbalances are common at certain times of life, such as in male babies (where maternal estrogen may influence the baby’s system), during puberty and as men get older. All of these imbalances are usually completely normal, and often need no treatment at all.

For babies and adolescents, the boobs usually disappear on their own as the hormone levels settle down.

Other causes of gynecomastia include:

  • Medicines (including some herbal skincare products)
  • Growths or tumours which produce hormones
  • Rare genetic conditions
  • Liver or thyroid disease

A big cause of gynecomastia is medication. There are several medicines which are prescribed for a range of problems that seem to cause gynecomastia. It may be possible to get rid of the man boobs by simply changing medications.

It is very important that this is done with the advice of a physician, as often a different treatment can be given to make sure the condition remains treated, but hopefully the gynecomastia goes away. Even if you think they are causing gynecomastia, prescribed medicines should not be stopped without talking to a doctor first, as this can be dangerous.

Some of the medicines which are linked to gynecomastia include those used to treat:

  • Stomach ulcers – including cimetidine
  • Depression -tricyclic antidepressants
  • Anxiety – Valium
  • Some heart problems – such as digoxin
  • Prostate cancer – including flutamide
  • Cancer – chemotherapy drugs
  • HIV – HAART medication

Some so-called ‘recreational’ or illicit drugs may also cause gynecomastia – including alcohol, marijuana, steroids taken for body-building, cannabis and methadone.

If a man develops gynecomastia and is using illicit drugs, he will be advised to stop taking them to see if this fixes the problem.

Swelling of the breast area can also be caused by the build-up of fatty tissue, called pseudogynecomastia. This is most often seen in men who are overweight or obese. Pseudogynecomastia is usually equal on both sides; it is very unusual to see pseudogynecomastia affecting one side only.

One-sided gynecomastia

One-sided gynecomastia – Other causes

One-sided swelling is less common than breasts on both sides. Although it is normally caused by the harmless enlargement of normal breast tissue there are a couple of conditions which might need more urgent treatment.

It is crucial, therefore, that a man with asymmetric or one-sided boobs visit a doctor to get them checked out. In very rare cases, one-sided breast swelling can be caused by conditions which need urgent treatment.

Breast tissue can become infected, which can cause the tissue to become swollen and may cause an abscess (or collection of pus) to form in the breast. If this happens the swelling may develop quite quickly and is usually very sore to the touch.

The man may feel unwell with a fever, and the breast itself can feel hot to the touch. A breast infection or abscess is usually quite easy to treat, either with antibiotics or an operation to release the pus.

Treatment is usually easier if it is started quickly, so it is important to get it checked out.

It is also possible for breast cancer to develop in men. If this happens it usually affects only one side, so can be seen as unilateral breast swelling. It can be difficult to tell the difference between simple breast tissue swelling and breast cancer, so it is vital to be seen by a doctor.

Cancer is usually hard to the touch, feels ‘fixed’ to either the skin or chest wall and may cause changes in the nipple. There is sometimes fluid or blood which leaks out of the nipple, or even sometimes through an ulcer on the skin. If any of these things are present, the advice of a doctor should be sought as soon as possible.

If the doctor thinks the swelling may be cancer, they will organise tests such as a mammogram, other scans or possibly a biopsy of the mass to confirm the diagnosis and plan treatment. Breast cancer can be treated, and outcomes are always better if it is caught early.

Again, it is crucial to seek medical advice as soon as possible.

Treatment for unilateral gynecomastia in males

In some cases, treatment may not be needed for unilateral gynecomastia. Simple advice and reassurance may be all that is needed – particularly if the swelling is minor, as often it will settle down on its own given time.

If a man is particularly keen to get rid of the swelling, treatment is often possible. Firstly a doctor will ask for a full list of all medicines and other drugs the man is taking. It may be that a change in medication can help reduce the one-sided swelling.

The doctor will also perform a full examination to check if the swelling really is breast tissue as well as examining other areas such as lymph nodes and testicles. It is possible that some tests or scans might be needed, but often this is not the case.

There are medications which might help reduce one-sided breast swelling. These are hormonal treatments which either reduce estrogen levels or try and increase testosterone.

There is no significant evidence that these medicines work well, and they often have negative side-effects, so they are not regularly used.

Unilateral gynecomastia surgery

Before and after surgery

Unilateral gynecomastia surgery

The most commonly offered treatment for gynecomastia affecting one side only is surgery. Unilateral gynecomastia surgery in males involves a mastectomy – an operation to remove the breast tissue, which is sometimes done alongside liposuction to remove any excess fatty tissue.

The surgery is usually carried out under general anaesthetic, with an incision under the nipple (or occasionally small ‘keyhole’ incisions across the chest). After the operation there will be some pain or discomfort which may be controlled with painkillers.

The surgical team will discuss with the man what to expect after the procedure, including how long he will need to stay in the hospital and when he can return to normal activity.

The surgery is usually very safe, although problems and complications such as infection or scar overgrowth are possible. In the end, most men are satisfied with the outcomes of the surgery.

Many people might think that operating on only one side is easier than treating both sides, but in fact this may not be true. The problem with operating on only one side is trying to achieve a good cosmetic appearance at the end of the procedure.

The man will still have breast tissue on the other side, and if all breast tissue and some fat is removed during the operation, he may feel that the appearance is still uneven after the procedure.

It is really important to discuss this with the surgeon who will be carrying out the operation, and perhaps to look at some ‘before and after’ photographs of similar operations to make sure the man’s expectations are realistic and to avoid disappointment.


Asymmetric or one-sided gynecomastia is less common than two-sided swelling. It can be particularly distressing as one-sided swelling is sometimes more noticeable.

Gynecomastia on one side can be caused by breast tissue responding to hormonal changes, prescribed medications or illicit drugs.

In rare cases, one-sided swelling can be caused by problems such as infection or cancer, and it is vital to seek medical advice. Unilateral gynecomastia is often treated with surgery.

It is important to have realistic expectations of the surgical outcomes, as it can be difficult to make the chest look completely symmetrical.

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