Why Are My Underarms Swollen?
The honest verdict
This is usually not an allergy
Swollen underarms are rarely caused by an allergy. The most common cause is enlarged lymph nodes (lymphadenopathy) responding to an infection elsewhere in the body — such as a cold, flu, or skin infection on the arm or chest. Other frequent causes include irritated or infected hair follicles (folliculitis), deodorant or antiperspirant irritation, and cysts. True allergic reactions can cause underarm swelling as part of a broader allergic response (hives, angioedema), but isolated underarm swelling without other allergy symptoms is far more likely to have a non-allergic cause.
This is a general research summary, not a diagnosis. It can't tell you what's causing your symptom — a clinician can.
Check this first
When swollen underarms is an emergency
Most of the time this can be handled calmly. But a few signs mean you shouldn't wait. Match what you're seeing to the right level of care below.
If any emergency sign below is present, call 911 or go to the ER now.
- Emergency
Swelling that appears suddenly with hives, itching, facial or lip swelling, or trouble breathing — call 911 (possible anaphylaxis).
Call 911 or go to the ER now
- Urgent
Swollen underarm with red streaking spreading from the area, warmth, fever, or chills — go to urgent care or ER (possible cellulitis or spreading infection).
Get seen today — urgent care or same-day visit
- Urgent
Painful, rapidly enlarging lump with fever and pus drainage — see a clinician promptly (possible abscess needing drainage).
Get seen today — urgent care or same-day visit
- See a doctor
A firm, painless, or hard lump that does not go away after 2-3 weeks, or is growing — see a clinician (needs evaluation to rule out lymphoma or other serious conditions).
Book a routine visit with a clinician
- See a doctor
Swollen underarm accompanied by unexplained weight loss, night sweats, or persistent fatigue — see a clinician (possible systemic infection or lymphoma).
Book a routine visit with a clinician
When in doubt, err toward getting seen. These tiers are a general guide, not a substitute for a clinician's judgment about your specific situation.
Key facts
The underarm contains 20-40 lymph nodes that filter fluid from the arm, chest wall, and breast — they commonly swell during infections.
Contact dermatitis from deodorants affects up to 2% of the population, with fragrance ingredients being the most common allergen.
Hidradenitis suppurativa affects about 1% of the population and most commonly appears in the underarms, groin, and under the breasts.
What's causing it
The same symptom can come from very different places. Here are the usual suspects for Underarms (axillae) — the area beneath the shoulder joint, containing lymph nodes, sweat glands, hair follicles, and skin folds, tagged by mechanism and how common each one is — so you can see where an allergy actually fits.
Reactive lymph nodes (lymphadenopathy)
The underarm contains lymph nodes that swell when fighting an infection — most commonly a viral illness (cold, flu, mononucleosis) or a bacterial infection in the arm, hand, or chest wall. The swelling is usually tender, feels like a pea-sized or larger lump under the skin, and resolves as the infection clears.
Folliculitis or hidradenitis suppurativa
Inflamed or infected hair follicles in the underarm cause tender, red bumps that can look like pimples or boils. Hidradenitis suppurativa is a chronic condition where blocked sweat glands cause painful, recurrent lumps and tunnels under the skin. Both are more common in people who shave or use antiperspirants.
Deodorant or antiperspirant irritation
Fragrances, aluminum compounds, and other ingredients in deodorants and antiperspirants can irritate the sensitive underarm skin, causing redness, swelling, and tenderness. This is an irritant reaction, not a true allergy, and typically improves by switching to a fragrance-free or aluminum-free product.
Contact dermatitis (allergic)
A true allergic contact dermatitis to ingredients in deodorants, laundry detergents, or fabrics can cause underarm swelling with redness, itching, and a rash. Common allergens include fragrances, propylene glycol, and nickel in clothing fasteners. The rash is typically itchy and well-demarcated to the area of contact.
Cyst or lipoma
Epidermoid cysts (sebaceous cysts) and lipomas (benign fatty tumors) can develop in the underarm. Cysts may become infected and tender; lipomas are usually painless and soft. Neither is dangerous, but any new or growing lump should be evaluated by a clinician.
Vaccine reaction
Some vaccines — particularly flu, COVID-19, and tetanus shots — can cause temporary swelling of the underarm lymph nodes on the side of the injection. This is a normal immune response that typically resolves within a few days to two weeks.
Is this an allergy? Usually, no
It's a common assumption, so it's worth being clear about — and pointing you toward what's more likely going on.
Swollen underarms are usually not an allergy. The most common causes are reactive lymph nodes from infections, irritated hair follicles, or deodorant irritation. True allergic reactions can cause underarm swelling as part of a broader allergic response — such as hives (urticaria) or angioedema — but these almost always involve other areas of the body and symptoms like itching, redness, and sometimes trouble breathing. Contact dermatitis from deodorant ingredients is a true allergic reaction, but it typically causes an itchy rash with redness and swelling confined to the area of contact, not a lump. If underarm swelling occurs alone without other allergy symptoms, a non-allergic cause is far more likely.
Because an allergy is unlikely to be the driver, the most useful next step is a clinician who can look at the actual cause — not an allergy test.
Relief you can try at home
If your symptom is mild and you have no red-flag signs, these gentle steps often help. They soothe — they don't replace figuring out the cause.
- 1
Apply a warm compress
A warm, damp cloth applied to the swollen area for 10-15 minutes several times a day can help reduce pain and encourage drainage if an infection is present.
- 2
Switch to a gentle deodorant
If irritation from deodorant or antiperspirant is suspected, stop using it for a few days and switch to a fragrance-free, aluminum-free product. A baking soda-based or hypoallergenic deodorant may be less irritating.
- 3
Rest and monitor for other symptoms
If the swelling is from reactive lymph nodes due to an infection, rest, hydration, and treating the underlying illness (e.g., cold, flu) will help the swelling resolve. Check for fever, sore throat, or skin infections on the arm or chest.
- 4
Avoid shaving or waxing until resolved
Shaving or waxing irritated underarm skin can worsen folliculitis or hidradenitis suppurativa. Let the area rest until the swelling and tenderness subside.
Stop any step that makes things worse. If the symptom isn't easing within a few days, check in with a clinician.
When to see a doctor
See a clinician if underarm swelling does not improve within 2-3 weeks, is growing, or is accompanied by fever, chills, red streaking, or pus drainage. A firm, painless lump that persists or grows needs evaluation to rule out lymphoma or other serious conditions. Sudden swelling with hives, facial swelling, or trouble breathing is a medical emergency — call 911. If you have recurrent painful lumps in the underarm (possible hidradenitis suppurativa), a dermatologist can provide long-term treatment options.
❓Frequently Asked Questions
Yes — deodorants and antiperspirants can cause underarm swelling through irritation or a true allergic contact dermatitis. Fragrances, aluminum compounds, and propylene glycol are common triggers. The swelling is usually accompanied by redness, itching, and a rash. Switching to a fragrance-free, aluminum-free product often resolves the issue within a few days.
Worry if the lump is firm, painless, and does not go away after 2-3 weeks, or if it is growing. Also seek medical attention if you have unexplained weight loss, night sweats, persistent fever, or fatigue — these can signal lymphoma or a systemic infection. Tender, pea-sized lumps that come and go with illness are usually normal reactive lymph nodes.
Yes — shaving can cause small cuts, ingrown hairs, and folliculitis (inflamed hair follicles), leading to red, tender bumps that look like pimples. These are not allergies but localized infections or irritation. Using a clean razor, shaving in the direction of hair growth, and moisturizing afterward can help prevent this.
Hidradenitis suppurativa (HS) is a chronic skin condition that causes painful, recurrent lumps under the arms, in the groin, and under the breasts. It occurs when hair follicles and sweat glands become blocked and inflamed. HS is not an infection or allergy — it is an inflammatory condition that requires dermatologic management. It is often mistaken for boils or acne.
Yes — it is common for underarm lymph nodes to swell after vaccines given in the arm, particularly flu, COVID-19, and tetanus shots. This is a normal immune response showing the body is building protection. The swelling is usually mild, tender, and resolves within a few days to two weeks.
Medically reviewed by
Dr. Chet TharpeMD
Last reviewed July 2026
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Read moreThe best next step is a real clinician
This symptom is usually not driven by an allergy, so an allergy test won't give you the answer. A primary care doctor or the right specialist can examine what's actually going on and guide you from there.
Curex focuses on at-home allergy testing and immunotherapy, which isn't the right fit for this symptom — and never a substitute for medical care.
This content is for informational purposes only and does not constitute medical advice, diagnosis, or treatment. It cannot tell you what is causing your swollen underarms or whether it is an allergy. If you have any emergency sign — trouble breathing, fast-spreading swelling, sudden weakness or confusion, or severe pain — call 911 or go to the nearest emergency room. For anything that is new, worsening, or persistent, see a licensed healthcare provider. Curex focuses on at-home allergy testing and immunotherapy, which is not the appropriate care for this symptom and never a substitute for a clinician.