A lump in the armpit may occur due to inflammation or infection in the area, caused by folliculitis, boils, hidradenitis suppurativa, or infectious mononucleosis. Although it is not as common, an armpit lump can also be a sign of a tumor from breast cancer or lymphoma.

Depending on the underlying cause, a lump in the armpit may be present with other symptoms, such as increased heat in the area, redness, sensitivity, pus, fever, weight loss or night sweats.

If you notice a lump in the armpit, it is important to consult a general practitioner or dermatologist for assessment. Once the cause is identified, treatment can be initiated. This may include antibiotics, anti-inflammatories or surgery.

What causes a lump in the armpit?

The main causes of a lump in the armpit are:

1. Folliculitis

Folliculitis is characterized by inflammation of the hair follicle, which is the structure in the skin where the hair roots are located. Swelling in this area can make it difficult for the hair to come out on the surface of the skin, and an obstruction can lead to symptoms, such as one or several small bumps that are painful, itchy, red or yellow in color, and have discharge.

Armpit folliculitis, also known as ingrown hairs, are relatively common and are usually caused by skin friction when wearing tight clothing. These ingrown hairs can also appear after shaving or waxing, or due to a bacterial or fungal infection.

What to do: Folliculitis can be treated at home by cleansing the armpit with antiseptic soap. Worse cases may require anti-inflammatory or antibiotic creams, ointments or tablets prescribed by a dermatologist. You should avoid shaving or waxing the skin until the inflammation improves.

2. Boil

A boil is also caused by an infection at the root of the hair follicle, however, it is deeper and causes inflammation of the surrounding area. It leads to an even larger lump that is more red and produces a large amount of pus.

This pus-filled lump can grow over time, causing pain, increased heat in the area and sensitivity to touch. It can appear in the armpit or any other area of the body that sweats a lot or undergoes friction.

What to do: You should see a doctor for assessment and to determine whether the boil should be drained. Your doctor may prescribe topical or oral antibiotics as well as warm compresses.

When treating a boil and to prevent worsening, you should use antiseptic soap to cleanse the area daily. Clothing that comes in contact with the boil should also be clean. Avoid popping the boil, as this can worsen inflammation and infection, making it more difficult to treat. Read more about tips to heal a boil quickly and relieve symptoms.

3. Hidrosadenitis suppurativa

Hidradenitis suppurativa is characterized by inflammation of the sweat glands which block of the passage of sweat out of the gland. This can lead to the formation of painful lumps that leave scars on the skin.

What to do: A dermatologic assessment is necessary, as this condition is treated with antibiotic creams and/or corticosteroid injections in the armpit. In more serious cases, surgery may be necessary to remove the affected area and replace it with a graft.

The doctor will also advise you to keep the area clean, avoid wearing tight clothing and apply warm compresses to the area.

4. Sebaceous cyst

A sebaceous cyst is a lump that forms under the skin. It has a rounded shape, and is usually only a few centimeters in diameter, but it can increase in size over time. This cyst can be hard or soft, and is movable when palpated. It can appear in the armpit or in other areas of the body.

Sebaceous cysts are benign are usually caused by an obstruction in the sebaceous gland. This leads to sebum accumulating under the skin. Although a small cyst generally does not cause symptoms, cysts that becomes inflamed or infected can lead to pain, increased heat in the area, sensitivity or redness.

What to do: Treatment should be oriented by a dermatologist, and usually consists of applying warm water compresses for 15 minutes to the area and using anti-inflammatories. Inflamed or infected cysts may require drainage followed by a round of antibiotics.

5. Swollen lymph node

A lymph node can become swollen in the presence of inflammation or infection in the arm, chest or breast region. Lymph nodes are a part of the immune system, and can increase in size to produce more defense cells in order to fight off foreign invaders.

Most of the time, a swollen lymph node in the armpit is not concerning. They can appear for a variety of reasons, such as an ingrown hair, folliculitis, boil, and lymphadenitis. They can also be a sign of a systemic disease, such as lupus, rheumatoid arthritis, dermatomyositis or sarcoidosis or cancer, especially when they grow a lot or are located in other parts of the body.

Read more about swollen lymph nodes in the armpit.

What to do: Treatment should be oriented by a doctor and depends on the underlying cause of the swelling. The doctor may prescribe anti-inflammatories, antibiotics, antivirals, corticosteroids or biological therapy. If it is related to cancer, surgical removal of the node or tumor that is causing its swelling may be advised, followed by chemotherapy or radiation therapy.

6. Allergies

Allergies are an inflammatory reaction that arise due to an exaggerated response of the immune system. This reaction can be triggered by substances such as deodorants, creams, or even clothing and can lead to the appearance of a lump in the armpit, irritation, intense itching, bumps or red spots in the skin.

What to do: You should try to identify the allergy trigger and avoid exposure to it whenever possible. If symptoms do not improve, you should consult a dermatologist who can perform an allergy test to identify the trigger and, if necessary, advise treatment with allergy medications or corticosteroids.

7. Infections

Some infections, such as HIV, herpes simplex or mononucleosis, can cause enlarged lymph nodes, and the appearance of a lump in the armpit or in other areas of the body, such as the neck and back of the neck.

What to do: You should consult your doctor or infectious disease specialist to determine the type of infection causing the swollen lymph node. This will help to guide treatment, which may include the use of antiretrovirals (for HIV infections), antivirals, such as acyclovir or valacyclovir (for cold sore infections) or analgesics or anti-inflammatories to reduce swelling and fever.

8. Lipoma

A lipoma is a round, soft lump that forms under the skin and is made up of fat cells. It can appear in the armpit, neck, back, shoulders or any place on the body where fat cells are present.

Generally, lipomas do not cause pain, however, in some cases it can grow and press on nerves in the area, leading to pain, inflammation, redness or increased heat in the area.

What to do: Generally, no treatment is necessary for a lipoma, however, large or uncomfortable lipomas can be surgically removed.

9. Breast cancer

Breast cancer is one of the main types of cancer that affect women, but it can also affect men. Its early stages usually do not cause symptoms. The main sign that may indicate the presence of the tumor is the palpation of a hard lump in the breast.

Other symptoms may include swelling, breast pain that can radiate to the arm, redness, and discharge from the nipples. Read more about the signs of breast cancer and how they can present.

What to do: If you notice any changes to your breast, you should advise your doctor so that breast cancer can be ruled out. If confirmed, the doctor will consider treatment options such as surgery, chemotherapy, radiation, hormone therapy or biological therapy.

10. Lymphoma

A lump in the armpit can also be a sign of lymphoma, which is a type of cancer of the lymph nodes. It can lead to the appearance of a hard lump in the armpit, which does not disappear after 1 or 2 months and does not stop growing.

Generally, in this type of cancer, other symptoms may be present in addition to the lump in the armpit, such as fever, night sweats, excessive tiredness and weight loss for no apparent reason.

What to do: You should report this lump to your doctor so that he or she can order testing to rule out this type of cancer. If confirmed, treatment will depend on the type and stage it presents with, and may include chemotherapy or radiation therapy.