Pain under your armpit is usually caused by something minor, like an irritated hair follicle, a pulled muscle, or a swollen lymph node fighting off an infection. The armpit is a crowded intersection of lymph nodes, muscles, nerves, and sensitive skin, so several different problems can produce pain in the same spot. Here’s how to figure out what’s likely going on.
Swollen Lymph Nodes From Infection
Your armpit contains a cluster of lymph nodes, small bean-shaped glands that filter germs and other harmful substances. When your body is fighting an infection, these nodes can swell and become tender. The most common cause of swollen lymph nodes is a viral infection like the common cold. But strep throat, ear infections, skin infections like cellulitis, mononucleosis, and even an infected tooth can also trigger swelling and soreness in your armpit.
Swollen nodes from infection are typically tender to the touch, feel somewhat soft, and may move a little under your fingers. They often show up on both sides of the body. In most cases, the tenderness resolves within a couple of weeks once the underlying infection clears. If a node is hard, doesn’t move, keeps growing over several weeks, or appears on only one side without an obvious infection, that’s worth getting checked.
Muscle Strain
Several large muscles overlap in the armpit area. The latissimus dorsi, a broad muscle running from your mid-back to your upper arm bone, is a common culprit. It helps with pushing, pulling, throwing, rowing, and overhead movements. Strain in this muscle tends to flare when you extend your hands in front of you, raise your arms overhead, or throw something. The pectoral muscles on the front of your chest also wrap near the armpit and can cause pain in the same area after heavy lifting, push-ups, or pull-ups.
Muscle-related armpit pain usually feels like a dull ache that sharpens with specific movements. It won’t come with a visible lump, fever, or skin changes. Rest and avoiding the triggering motion for a few days is typically enough.
Skin Irritation and Contact Dermatitis
The skin in your armpit is thin, warm, and frequently exposed to products. Deodorants and antiperspirants are a leading cause of contact dermatitis in this area. Fragrances are the most common allergen in deodorant, followed by propylene glycol, a widely used ingredient that some people react to. Essential oils, lanolin, and parabens are other frequent offenders.
Contact dermatitis shows up as redness, itching, burning, or a patchy rash. It can feel raw or stinging, especially after applying a product. If you recently switched deodorants or shaving products and the pain started shortly after, the product is the likely cause. Stopping the product for a week or two usually resolves the rash.
Folliculitis
Shaving, friction from clothing, and sweat can irritate hair follicles in the armpit, leading to folliculitis. This looks like clusters of small pimples or red bumps around hair follicles. The bumps can be itchy, tender, and sometimes fill with pus that breaks open and crusts over. Bacterial folliculitis is the most common type, usually caused by staph bacteria that enter through tiny nicks or irritated follicles.
Mild folliculitis often clears on its own within a week if you keep the area clean and avoid shaving until it heals. Deeper or spreading infections may need treatment.
Hidradenitis Suppurativa
If you get painful lumps under the skin of your armpit that last for weeks or months, hidradenitis suppurativa is a possibility worth knowing about. This chronic condition typically starts with a single, painful bump under the skin that persists much longer than a normal pimple or ingrown hair. Over time, more bumps may appear in areas with sweat glands or skin-on-skin contact, including the armpits, groin, and buttocks.
Signs that distinguish it from a one-time bump include blackheads appearing in small, pitted clusters (often in pairs), lumps that break open and drain foul-smelling pus, and tunnels that form under the skin connecting different bumps. These tunnels heal slowly, if at all. If your armpit pain keeps returning in the same spot, makes it difficult to move your arm, or doesn’t improve within a few weeks, a dermatologist can evaluate whether this condition is the cause.
Nerve-Related Pain
A network of nerves called the brachial plexus runs through the armpit area, connecting the spinal cord to the arm and hand. Injury or compression of these nerves produces pain that feels distinctly different from a muscle ache or swollen node. The hallmark sensations are electric shock-like jolts, burning that shoots down the arm, and numbness or weakness in the hand, arm, or shoulder.
Minor nerve compression from sleeping in an awkward position or carrying a heavy bag usually resolves on its own. More serious injuries, which can happen from trauma, sports, or repetitive overhead work, may cause persistent weakness or loss of feeling in the arm.
Breast-Related Causes
Armpit pain can sometimes relate to breast tissue, which extends into the armpit area. Hormonal fluctuations during the menstrual cycle commonly cause tenderness that radiates into the armpit. This type of pain is cyclical, appearing before a period and easing once it starts.
An armpit lump in a woman should be evaluated by a healthcare provider, particularly if it’s hard, painless, fixed in place, or growing. Breast cancer can occasionally present as a lump in the armpit before any changes are noticed in the breast itself. To put this in perspective, the vast majority of armpit lumps turn out to be benign, but a persistent or unusual one warrants examination. Clinicians differentiate concerning lumps from harmless ones partly by checking whether the lump is tender (more common with infection) versus painless (which can signal something that needs further testing), and whether it’s symmetrical on both sides or isolated to one.
How to Tell What’s Causing Your Pain
A few distinguishing features can help you narrow things down:
- Tender, soft, movable lump with recent cold or illness: likely a swollen lymph node from infection.
- Pain that worsens with arm movement or exercise: likely muscular strain.
- Red rash, itching, or burning after using a product: likely contact dermatitis or folliculitis.
- Deep, persistent bump lasting weeks: could be hidradenitis suppurativa.
- Shooting, electric, or burning pain with numbness: likely nerve involvement.
- Cyclical tenderness tied to your period: likely hormonal.
Most armpit pain is temporary and resolves within one to two weeks. A lump that persists beyond two weeks without shrinking, pain that keeps getting worse, unexplained weight loss or night sweats alongside armpit swelling, or a hard, immovable lump on one side all warrant a medical evaluation.