What should you know about a lump with pus under the armpit?

Pus is a thick fluid made up of immune cells, germs, and damaged tissue. It can collect in a boil or skin abscess, sometimes after a hair follicle becomes blocked or infected. Shaving, friction, sweating, or an ingrown hair can irritate the skin in the armpit. Repeated painful lumps that drain, heal slowly, or leave scars can occur with hidradenitis suppurativa, a long-term inflammatory skin condition.

An enlarged lymph node can also cause a lump in the armpit, although a visible pus-filled centre points more toward a skin process. A breast or chest infection may make nearby lymph nodes swell. Breast cancer and lymphoma can also involve lymph nodes, but a pus-filled lump is not enough to identify either condition, and most armpit lumps have causes other than cancer. A physical examination is the safest way to distinguish these possibilities.

Keep the area clean and dry, and use a warm, clean compress for short periods to ease discomfort. Do not squeeze, pierce, or cut the lump; pressure can push infection deeper or spread it to nearby skin. Avoid shaving over it and choose loose clothing until the skin settles. Do not share towels, razors, or clothing that touches the area.

Arrange a medical review if the lump is painful, continues to enlarge, drains repeatedly, or does not begin improving. A clinician may decide whether it needs drainage, a swab, prescription treatment, or evaluation of a nearby breast, chest, or skin problem. Do not apply leftover antibiotics or steroid creams unless a clinician has advised you to use them.

Seek urgent medical care if redness or swelling is spreading quickly, the pain is severe, or you develop fever, chills, marked weakness, or red streaks extending from the lump. Prompt assessment is also appropriate if you have diabetes, a weakened immune system, or take medicines that suppress immunity.

Book an appointment soon for a firm lump that remains after the drainage settles, a lump that keeps returning, or swelling accompanied by an unexplained breast or chest change, night sweats, or unintentional weight loss. These findings do not diagnose cancer, but they should not be ignored.

You could ask: Does this look like an abscess, an infected follicle, hidradenitis suppurativa, or a swollen lymph node? Does it need drainage or a sample? What skin-care changes may reduce recurrence? Which changes should prompt an earlier review? If the lump remains, would I need an examination of the breast or chest or further testing?