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Painful Lump in the Armpit: Causes and Warning Signs
By Your Health Magazine Health Information Team
A knot in the underarm with pain is commonly caused by a swollen lymph node, irritated hair follicle, cyst, boil, or other skin inflammation. Pain and tenderness often point to infection or irritation, but they cannot identify the cause by themselves. An unexplained lump should be medically evaluated if it is growing, hard, persistent, or accompanied by breast changes, fever, night sweats, or unexplained weight loss.
Common Causes of a Painful Armpit Lump
The armpit contains lymph nodes, hair follicles, sweat glands, fatty tissue, blood vessels, and nerves. A problem involving any of these structures can feel like a lump or knot.
| Possible cause | Typical clues |
|---|---|
| Swollen lymph node | A tender lump deeper under the skin, sometimes following an infection, vaccination, or inflammation involving the arm, chest, or breast |
| Ingrown hair or folliculitis | A small, surface-level bump after shaving or friction, sometimes with redness, itching, or a visible hair |
| Boil or skin abscess | An increasingly painful, warm, red lump that may become soft in the center or drain pus |
| Skin cyst | A round lump beneath the skin that may become painful if inflamed or infected |
| Hidradenitis suppurativa | Deep, painful lumps that recur in the armpits or other skin-fold areas and may drain, form tunnels, or leave scars |
| Breast or lymphatic condition | A persistent or firm underarm lump, especially with a breast lump, nipple change, or change in breast skin |
Swollen lymph nodes
Lymph nodes help filter lymphatic fluid and participate in the immune response. Underarm nodes may enlarge when the body is reacting to a nearby skin wound, an infection in the arm or breast, or certain illnesses affecting the whole body. Recent vaccination can also cause temporary lymph-node swelling on the vaccinated side. The MedlinePlus overview of armpit lumps lists swollen nodes, infections, cysts, medications, vaccinations, and less common cancers among the possible causes.
Boils, ingrown hairs, and abscesses
A lump close to the skin’s surface is more likely to involve a hair follicle or sweat-producing area. Shaving can irritate the skin or contribute to ingrown hairs. Bacteria may then infect a follicle or damaged skin, producing a boil or abscess. These lumps are often red, warm, and painful, and some eventually drain fluid or pus.
Hidradenitis suppurativa
Repeated boil-like lumps deserve particular attention. Hidradenitis suppurativa symptoms commonly begin with tender, deep nodules in areas where skin touches skin, including the armpits. The lumps may return in the same place, connect beneath the skin, drain, or cause scarring. A dermatologist can distinguish this chronic inflammatory condition from ordinary acne, cysts, and infections.
Can a Painful Underarm Lump Be Cancer?
Most painful armpit lumps are not cancer. Infection and inflammation are more common explanations, particularly when the area is tender, red, or warm. Cancer-related lymph nodes are often described as painless, firm, or fixed, but there are exceptions. Pain alone cannot rule cancer in or out.
A lump under the arm can occasionally be associated with breast cancer, lymphoma, or another cancer affecting lymph nodes. Review the area for a breast lump or thickening, nipple discharge, nipple retraction, dimpling, persistent redness, swelling, or a change in breast shape. The National Cancer Institute advises discussing unusual breast or underarm changes with a healthcare professional, even after a recent normal mammogram. Breast cancer can occur in people of any sex.
Symptoms such as drenching night sweats, an unexplained fever, persistent fatigue, unexplained weight loss, or enlarged nodes in several parts of the body also require medical evaluation. These symptoms can have noncancerous causes, but they should not be ignored.
What You Can Do at Home
If the lump is small, recently developed, and not accompanied by warning signs, gentle measures may reduce discomfort while you arrange care or monitor for improvement:
- Do not squeeze, puncture, or repeatedly press the lump. This can worsen inflammation or push infection deeper into the skin.
- Pause shaving and avoid products that sting or irritate the area.
- Wear loose clothing to reduce friction.
- Apply a clean warm compress to a boil-like bump. A cool compress may be more comfortable for general swelling or tenderness. Learn more about using cold compresses for swollen lymph nodes.
- If you normally can take an over-the-counter pain reliever safely, follow its package directions. Ask a pharmacist or clinician if you have kidney disease, liver disease, stomach ulcers, take blood thinners, are pregnant, or are unsure which product is appropriate.
Home care should not replace evaluation of an unexplained or worsening lump. Antibiotics are helpful only for certain bacterial infections and should not be taken without an appropriate diagnosis.
When to Seek Care
Contact a primary care clinician, gynecologist, dermatologist, or other qualified healthcare professional if:
- The lump is unexplained, enlarging, or lasts longer than about two weeks.
- It is hard, irregular, or does not move when gently touched.
- Pain, redness, warmth, or swelling is getting worse.
- The lump repeatedly returns, drains, or leaves scars.
- You notice a breast lump, nipple discharge, skin dimpling, or another breast change.
- You have persistent fever, night sweats, unusual fatigue, or unexplained weight loss.
- You have a weakened immune system, are receiving cancer treatment, or have poorly controlled diabetes.
Seek prompt same-day care for rapidly spreading redness, severe pain, red streaks, significant pus drainage, fever with worsening swelling, or a painful breast and fever while breastfeeding. Emergency help may be necessary if swelling is accompanied by trouble breathing, fainting, confusion, or other signs of severe illness.
How an Underarm Lump Is Evaluated
A clinician will ask when the lump appeared, whether it has changed, and whether there was a recent illness, vaccination, skin injury, shaving irritation, or breast symptom. The examination may include the skin, lymph nodes, arm, chest, and breast tissue.
Depending on the findings, testing may include an ultrasound, blood tests, breast imaging, a sample of drainage, or a biopsy. Testing does not automatically mean cancer is suspected; it helps determine whether the lump contains fluid, represents an enlarged lymph node, or needs closer examination.
For reliable general information while preparing for an appointment, the National Library of Medicine provides a searchable collection of MedlinePlus health topics. Avoid trying to diagnose a painful underarm knot by feel alone, because several conditions can look and feel similar.
This article provides general education and is not a diagnosis or a substitute for care from a qualified healthcare professional.
