A painful lump in your armpit is most often a swollen lymph node reacting to a nearby infection, an inflamed hair follicle, or a cyst. Pain is actually a reassuring sign in most cases: it usually points to an active immune response or a skin issue rather than something more serious. Understanding the most likely causes can help you figure out what you’re dealing with and whether it needs medical attention.
Swollen Lymph Nodes Are the Most Common Cause
Your body has roughly 800 lymph nodes, and a significant cluster of them sits in each armpit. These small, bean-shaped glands filter fluid and trap bacteria, viruses, and other threats. When your immune system is fighting something off, the nodes nearest to the problem swell, and that swelling often hurts. Pain and tenderness are hallmarks of lymph node inflammation.
The infections behind a painful armpit node are usually routine. Common colds, the flu, sinus infections, and minor skin infections on your hand, arm, or chest can all trigger swelling in armpit nodes because those nodes drain fluid from the upper body. Bacterial infections from staph or strep are frequent culprits. Mononucleosis, caused by the Epstein-Barr virus, is another well-known trigger. Less commonly, chickenpox, shingles, measles, or hepatitis can cause widespread lymph node swelling that includes the armpits.
Infection-related swelling typically shrinks on its own once the underlying illness clears. Over-the-counter pain relievers can help with discomfort in the meantime. If the lump persists for more than two weeks, keeps growing, or you develop a fever without an obvious cause, that warrants a closer look.
Infected Hair Follicles and Boils
The armpit is a warm, moist environment full of hair follicles, which makes it prime territory for folliculitis. This is an infection of the hair follicle itself, and it shows up as clusters of small red bumps or pus-filled blisters around the follicle opening. The skin feels tender, itchy, and sometimes burns. Shaving, friction from tight clothing, and heavy sweating all raise the risk.
Superficial folliculitis stays near the skin’s surface and often resolves without treatment. Deep folliculitis, however, involves the entire follicle and can progress into a boil (also called a furuncle), which is a larger, deeply infected lump filled with pus. A boil tends to appear suddenly as a painful, inflamed bump that grows over several days before it either drains on its own or requires medical drainage. A cluster of connected boils is called a carbuncle and is more severe. Staph bacteria are the usual cause.
Warm compresses applied for 10 to 15 minutes several times a day can help a boil come to a head and drain. Resist the urge to squeeze it, as that can push the infection deeper. Antibiotics are sometimes necessary if the infection spreads or doesn’t resolve.
Cysts in the Armpit
Cysts are fluid-filled or semi-solid sacs that develop under the skin, and they’re common in the armpit. They typically feel round, smooth, and movable under your fingers. A cyst may sit quietly for months or years and only become painful when it gets inflamed or infected, at which point it can swell rapidly and become red and tender.
An infected cyst sometimes needs to be drained by a healthcare provider and may require a short course of antibiotics. Cysts that keep coming back or cause repeated problems can be surgically removed. On their own, cysts are benign, but because they can look and feel similar to other types of lumps, getting a persistent one evaluated is worthwhile.
Hidradenitis Suppurativa
If painful armpit lumps keep returning in the same area, hidradenitis suppurativa (HS) is a possibility worth knowing about. This chronic skin condition targets areas where skin rubs together, with the armpits and groin being the most common sites. It causes deep, painful lumps that become inflamed, grow larger, and eventually break open, draining fluid and pus. The drainage can have an unpleasant odor.
HS exists on a spectrum. Mild cases involve one or a few lumps confined to a single area. Moderate cases involve recurring lumps that break open and appear in more than one body region. Severe HS causes widespread lumps, scarring, and chronic pain that can limit movement. Early signs also include small pitted areas of skin with blackheads. Mild HS frequently progresses to moderate disease if untreated, so early recognition matters. The condition is manageable with medical help, but it doesn’t resolve on its own.
Vaccine Reactions
If you recently received a vaccination in your upper arm, a swollen armpit node on that same side is a well-documented side effect. This is your immune system responding to the vaccine exactly as intended. Research on COVID-19 vaccines found that about 9% of recipients developed noticeable lymph node swelling in the armpit, with a median onset of roughly 1.7 weeks after the second dose. Women were affected more often than men (12.5% versus 3.4%).
Vaccine-related swelling is typically mild. In studies, the largest swollen nodes measured around 7 millimeters across, roughly the size of a small pea. This type of swelling resolves on its own, usually within a few weeks, and doesn’t require treatment.
Breast Tissue in the Armpit
Something many people don’t realize: breast tissue can extend into the armpit. This is called accessory breast tissue, and it’s a normal anatomic variation that’s frequently present on both sides. Most people with it never notice. But because this tissue responds to hormones, it can become swollen and painful during pregnancy, breastfeeding, the premenstrual phase of your cycle, or even during puberty. When this happens, it creates a palpable lump with pain, swelling, and sometimes redness.
During a clinical exam, accessory breast tissue is commonly mistaken for a swollen lymph node, a cyst, or even a tumor. If you notice that an armpit lump seems to fluctuate with your menstrual cycle, hormonal breast tissue is a likely explanation. Imaging can confirm it.
When a Lump Needs Further Evaluation
Most painful armpit lumps are benign, but certain features warrant prompt medical attention. A lump that is hard, fixed in place (doesn’t move when you push it), and keeps growing over weeks is more concerning than one that’s soft, mobile, and tender. Painless lumps actually deserve more scrutiny than painful ones, since cancerous lymph nodes tend to be firm and painless rather than inflamed and sore.
Other signals to take seriously include a lump that persists longer than two weeks without any obvious infection, unexplained weight loss, night sweats, or lumps appearing in multiple locations at once (like both armpits plus the neck or groin). A history of breast cancer also changes the calculation, since armpit nodes are a common site for breast cancer to spread.
When imaging is needed, ultrasound of the armpit is the standard first step. It can distinguish a swollen lymph node from a cyst, abscess, or solid mass and evaluate features like shape and blood flow patterns. If the ultrasound findings are inconclusive or suspicious, a needle biopsy of the node can provide a definitive answer. This is typically done with ultrasound guidance and involves either a fine needle aspiration or a core biopsy, both of which are outpatient procedures.
Managing a Painful Armpit Lump at Home
For lumps that appear alongside a cold, flu, or minor skin infection, a period of watchful waiting is often the right approach. Warm compresses can ease pain and help infected lumps drain. Over-the-counter anti-inflammatory medications reduce swelling and discomfort. Avoid irritating the area with heavy deodorant, aggressive shaving, or tight clothing while the lump is present.
If the lump doesn’t start improving within one to two weeks, gets noticeably larger, turns very red and hot, or you develop a fever, those are signs the issue needs professional evaluation. Antibiotics may be necessary for bacterial infections, and abscesses or persistent cysts sometimes need to be drained or removed.