Armpit pain has a wide range of causes, from something as simple as a razor nick or irritating deodorant to swollen lymph nodes fighting off an infection. The armpit (axilla) is a dense crossroads of lymph nodes, nerves, blood vessels, muscles, and sensitive skin, which means many different problems can produce pain in this one small area. Most causes are temporary and harmless, but a few warrant prompt medical attention.
Swollen Lymph Nodes
Your armpits contain a large cluster of lymph nodes, small filtering stations that trap bacteria, viruses, and other foreign material. When your immune system is actively fighting something off, these nodes can swell and become tender. This is one of the most common reasons for armpit pain, and it often shows up alongside a cold, flu, or other obvious illness.
Bacterial skin infections on the hand, arm, or chest are a frequent trigger because the lymph fluid from your entire upper body drains through the axillary nodes. Cat-scratch disease, staph or strep skin infections, and mononucleosis (caused by the Epstein-Barr virus) are well-known culprits. Other infections linked to axillary lymph node swelling include HIV, cytomegalovirus, tuberculosis, and even some sexually transmitted infections like syphilis. Routine vaccinations, particularly COVID-19 and flu shots given in the upper arm, can also cause temporary node swelling that lasts a few days to a few weeks.
Skin Irritation and Contact Dermatitis
The skin in your armpit is thin, frequently moist, and exposed to friction from clothing and arm movement. That combination makes it especially vulnerable to irritation. Contact dermatitis, an inflammatory reaction triggered by a chemical touching your skin, is a common source of burning or stinging armpit pain.
Fragrance is the leading contact allergen in deodorants. Specific fragrance chemicals like hydroxycitronellal, eugenol, and geraniol are among the most likely to cause a reaction. Propylene glycol, a solvent and stabilizer used in many personal care products, is the second most common contact allergen in deodorants. Its concentration in some formulas can reach as high as 73%, making it both an irritant and an allergen. Aluminum compounds, the active ingredient in most antiperspirants, occasionally cause reactions as well, though true aluminum allergy is uncommon.
Beyond what you apply directly, textile dyes in clothing (especially synthetic fabrics) and fragrances or preservatives in laundry detergent can irritate armpit skin. If your pain comes with redness, itching, or a rash that lines up with where your shirt seam sits or where you apply deodorant, a product switch is a reasonable first step.
Folliculitis and Infected Hair Follicles
Shaving, waxing, or plucking armpit hair can damage hair follicles and let bacteria in. The result is folliculitis: small, red, painful bumps that may fill with pus. Mild cases often clear up on their own with warm compresses and a break from shaving. A benzoyl peroxide wash used for five to seven days while showering can help. Persistent or spreading bumps may need a prescription topical antibiotic. If infections keep coming back, a healthcare provider may test for antibiotic-resistant bacteria like MRSA.
Hidradenitis Suppurativa
If you get recurring painful lumps under the armpit skin that last weeks or months, hidradenitis suppurativa (HS) is a possibility worth exploring. HS causes small, tender bumps that fill with pus, typically in areas where skin rubs together: armpits, groin, buttocks, and under the breasts. It usually starts with a single pea-sized lump that persists far longer than an ordinary pimple.
Over time, HS can progress. Bumps may break open and drain foul-smelling pus. Blackheads can appear in pairs in small pitted areas of skin. In more advanced cases, tunnels form under the skin connecting the lumps, and these wounds heal very slowly. HS is a chronic condition, not a one-time infection, so early diagnosis matters for managing it before scarring develops.
Muscle Strain and Nerve Injury
The axillary nerve runs from your neck through the brachial plexus (a nerve network in your shoulder), under your collarbone, and behind the top of your upper arm bone. It controls movement and sensation in the shoulder and upper arm area, including pain and temperature signals. Overuse injuries, heavy lifting, or a direct blow to the shoulder can irritate or compress this nerve, producing pain that radiates into the armpit.
A pinched nerve in the neck (cervical radiculopathy) can also refer pain down into the shoulder and armpit, even though the source of the problem is nowhere near the arm. Muscle strains in the chest (pectoralis) or the latissimus dorsi, which forms part of the rear wall of the armpit, are another straightforward cause. These typically improve with rest, gentle stretching, and over-the-counter pain relief.
Referred Pain From the Heart
Left armpit pain specifically can, in rare cases, be related to the heart. Classic cardiac pain presents as a crushing sensation behind the breastbone that radiates to the left arm, shoulder, neck, or jaw. This happens because the sensory nerves from the heart connect to the same spinal cord segments (the upper five thoracic roots) as nerves from the chest, shoulder, and arm. Your brain can misinterpret where the signal is coming from.
Left armpit pain alone, without chest pressure, shortness of breath, or other cardiac symptoms, is very unlikely to be heart-related. But if armpit or left arm pain comes on suddenly alongside tightness in the chest, lightheadedness, or nausea, treat it as an emergency.
Lumps: Benign vs. Concerning
Finding a lump in your armpit is alarming, but most armpit lumps are benign. Swollen lymph nodes from infection, lipomas (soft fatty lumps), and cysts are far more common than cancer. Size ranges from as small as a pea to as large as a golf ball.
A few physical characteristics help distinguish harmless lumps from ones that need urgent evaluation. Cancerous lumps tend to be hard, painful, and fixed in place, meaning they don’t move when you press on them. Benign lumps are more likely to feel soft or rubbery and shift slightly under your fingers. These are rough guidelines, not a diagnosis. Any lump that persists for more than two weeks, keeps growing, or appears without an obvious cause like a recent infection or vaccination deserves a professional look.
How Armpit Pain Is Evaluated
For a visible lump, ultrasound is the standard first imaging test. It can distinguish a fluid-filled cyst from a solid mass and evaluate lymph node structure. On ultrasound, features that raise concern for malignancy include a lymph node wider than 1 cm on its short axis, cortical thickness greater than 3 mm, and the absence of a normal fatty center (called a fatty hilum). That last finding alone carries a 90% to 93% chance of indicating malignancy when present.
If ultrasound reveals something suspicious, a needle biopsy can be performed at the same appointment for a definitive answer. For women, a diagnostic mammogram often accompanies axillary ultrasound to check for an underlying breast lesion. CT or MRI scans are reserved for more complex situations, such as suspected chest wall involvement or when a primary cancer source needs to be identified.
When no lump is present and the pain seems related to skin irritation, muscle strain, or nerve issues, imaging is usually unnecessary. A physical exam and history are often enough to point toward the right cause.