Dark lump under the armpit

The colour may reflect inflammation, bruising, a blocked hair follicle or a skin growth, while a deeper swelling may be an enlarged lymph node. Colour and touch alone cannot establish the diagnosis.

Tender surface lumps often arise from an infected follicle, boil, cyst or recurring inflammation around sweat glands. Lymph nodes can enlarge when the arm, breast or nearby skin is infected or inflamed. A dark mark may remain after irritation has settled.

Breast cancer can present with an armpit lump, sometimes alongside a breast change, nipple change or swelling. Lymphoma can present with enlarged lymph nodes that are often not painful. These presentations make assessment necessary; they do not mean that an armpit lump is cancer.

Arrange a primary care or breast clinic examination if the lump does not settle, keeps returning or is changing. Note when it appeared, whether it is painful, whether it moves beneath the skin, and whether shaving, deodorant, illness or vaccination preceded it. Check for changes in the breast, chest wall and arm without repeatedly squeezing the lump.

A clinician may inspect the skin, examine nearby lymph node areas and breast tissue, and decide whether ultrasound or another test is appropriate. Avoid piercing or attempting to drain the lump at home because this can worsen infection and obscure its appearance.

Seek prompt medical review if the lump is hard, fixed, enlarging, repeatedly returns, or occurs with a breast lump, nipple discharge, breast skin dimpling, unexplained fever, drenching night sweats or unintended weight loss.

Get urgent help if redness spreads quickly, pain becomes severe, pus is accompanied by fever, the arm becomes markedly swollen, or you feel generally very unwell. These features can indicate an infection needing timely treatment.

Ask whether the lump seems to arise in the skin or a lymph node, whether nearby breast tissue should be imaged, and which changes should bring you back sooner. If infection is suspected, ask how to care for the area and how improvement will be assessed.