Pain under the left armpit usually stems from something local and treatable, such as a strained muscle, an irritated lymph node, or a skin condition, rather than something dangerous. The armpit is a dense crossroads of muscles, nerves, lymph tissue, blood vessels, and skin glands, so a surprising number of problems can produce pain in that exact spot. Because the left side sits close to the heart, though, that location understandably triggers worry. Understanding the most common causes and knowing which warning signs actually warrant a trip to the emergency room can save you both unnecessary panic and genuine risk.
Muscle Strain, Especially the Serratus Anterior
The single most common reason for underarm pain that appears “out of nowhere” is a muscular issue. Several muscles fan through the armpit region, including the pectoralis major across the chest, the latissimus dorsi wrapping from the back, and the serratus anterior, which anchors along the ribs beneath your shoulder blade. The serratus anterior is easy to aggravate with overhead movements like painting a ceiling, lifting heavy boxes, or even an intense round of swimming or rock climbing. Once irritated, it can produce a burning or stabbing sensation that starts along the side of the chest wall and radiates toward the shoulder and arm, sometimes mimicking something more alarming.
A case study published in Pain Medicine described exactly this pattern: a patient with constant burning, stabbing pain on the left chest wall radiating to the left shoulder and arm. The culprit turned out to be discrete tender points over the serratus anterior muscle group, and the pain could be reproduced simply by pressing on those points during a physical exam.1Pain Medicine. Serratus Anterior Muscle Pain Syndrome: A Diagnostic Conundrum That “reproducible by palpation” quality is actually helpful: if pushing on the sore area recreates the pain, a muscular cause is far more likely than a cardiac one.
Rest, over-the-counter anti-inflammatories, and gentle stretching resolve most muscle-related armpit pain within a week or two. If you recently started a new workout, moved heavy furniture, or slept in an awkward position, a pulled or overworked muscle is the explanation worth considering first.
Swollen Lymph Nodes
Your armpit contains a cluster of lymph nodes that act as filters for the immune system. When your body fights off an infection in the arm, chest, or even a cut on your hand, those nodes can swell, become tender, and create a noticeable ache under the arm. Common colds, skin infections, and even a fresh tattoo on that side of the body can trigger this response. The swelling is usually temporary and resolves once the underlying infection clears.
Vaccinations are another well-documented trigger. After the rollout of COVID-19 boosters, breast oncologists began reporting cases of axillary lymph node swelling that showed up on imaging and caused concern. One case documented swollen axillary lymph nodes visible on ultrasound just 14 days after a third COVID-19 vaccination.2PubMed Central. Axillary Lymph Node Swelling After COVID-19 Booster Vaccination: Japanese Case Report and Literature Review Flu shots and other vaccines administered in the upper arm can produce the same temporary effect. If you recently received a vaccination on the left side, a sore, swollen node under that arm is a normal immune reaction and not a sign of disease.
A lymph node that stays enlarged for more than two weeks after an infection has resolved, grows steadily without an obvious cause, or feels hard and fixed in place rather than soft and movable deserves a visit to your doctor. In rare cases, persistent lymph node enlargement can signal lymphoma or metastatic cancer, but isolated armpit pain without other symptoms is a poor predictor of malignancy on its own.
Skin Conditions in the Armpit
The armpit is warm, moist, and constantly in contact with friction and products, making it prone to skin problems that can feel surprisingly painful.
Hidradenitis Suppurativa
Hidradenitis suppurativa is a chronic inflammatory skin condition that produces painful, deep-seated nodules or boils in areas rich in apocrine sweat glands, and the armpit is one of its favorite locations.3Postgraduate Medical Journal. Hidradenitis suppurativa: a common and burdensome, yet under-recognised, inflammatory skin disease It typically begins after puberty and tends to relapse in cycles. Over time, the nodules can progress to abscesses, tunneling tracts under the skin, and scarring.4PubMed Central. Presentations of Cutaneous Disease in Various Skin Pigmentations: Hidradenitis Suppurativa If you keep getting painful lumps under one or both armpits that drain and return, hidradenitis suppurativa is worth discussing with a dermatologist. It is frequently misdiagnosed as simple boils or ingrown hairs, and many people live with it for years before getting the right diagnosis.
Contact Dermatitis from Deodorants
Deodorants and antiperspirants are among the most common cosmetic triggers for contact dermatitis, particularly in people sensitive to fragrance. In a clinical provocation study, 60 percent of tested deodorants caused eczema in fragrance-sensitive individuals when applied to the armpit.5Contact Dermatitis. Deodorants: a clinical provocation study in fragrance-sensitive individuals The resulting rash can burn, itch, and ache, especially if the skin cracks or becomes secondarily infected. Switching to a fragrance-free product or going without deodorant for a few days is the simplest way to test whether your product is the problem. Razor burn and ingrown hairs from shaving compound the issue further, creating a painful combination in skin that is already irritated.
Nerve-Related Armpit Pain
The intercostobrachial nerve runs through the armpit and supplies sensation to the inner upper arm. When this nerve is irritated or damaged, it can produce burning, shooting, or electric-shock-type pain in the axilla and down the inner arm. The most well-documented cause is surgical: axillary lymph node dissection during breast cancer treatment frequently injures the intercostobrachial nerve, giving rise to what clinicians sometimes call post-mastectomy pain syndrome. One series described patients with pain in the axilla, inner upper arm, and shoulder, all of whom had evidence of intercostobrachial nerve damage on neurological examination.6Pain. Clinical section Post-axillary dissection pain in breast cancer due to a lesion of the intercostobrachial nerve Intercostobrachial neuralgia is in fact considered a common source of this postoperative pain.7PubMed. Ultrasound-Guided Intercostobrachial Nerve Block for Intercostobrachial Neuralgia in Breast Cancer Patients: A Case Series
You do not need to have had surgery, however, to develop nerve pain in the armpit. Compression from sleeping with your arm pinned under your body, a heavy bag strap pressing into the area repeatedly, or even a tight-fitting garment can irritate local nerves enough to produce discomfort. Nerve-related pain often has a distinctive quality: tingling, numbness, or a sensation that radiates along a path rather than staying in one spot. If those descriptors match what you are feeling, a nerve issue is worth exploring with your doctor.
Hormonal and Breast-Tissue Causes
Breast tissue does not stop neatly at the border of the breast. A tail of tissue known as the axillary tail of Spence extends up toward the armpit in many people, and some individuals have ectopic breast tissue that sits fully within the underarm. This tissue responds to the same hormonal shifts as the breast itself, which means it can swell and ache with menstrual cycles, during pregnancy, or in the early days of breastfeeding.
When ectopic axillary breast tissue becomes symptomatic, it usually does so during pregnancy or immediately postpartum, when a woman begins breastfeeding. The primary complaint is swelling and pain from engorgement, and the standard approach is conservative management: allowing the tissue to regress on its own once breastfeeding slows or stops.8Mayo Clinic Proceedings. Breast-feeding with ectopic axillary breast tissue If you are pregnant or recently gave birth and have a painful, swollen area under one arm, this is a common and benign explanation.
Outside of pregnancy, cyclical breast pain that flares before your period and settles afterward can extend into the armpit because of that axillary tail. This tends to affect both sides but can feel worse on one side depending on your anatomy. A well-fitting bra that does not dig into the underarm can reduce mechanical irritation of already-tender tissue.
When Left-Sided Pain Could Be Coming from Somewhere Else
Referred pain is discomfort felt in one location that actually originates elsewhere. Two referred-pain scenarios are worth knowing about when the left armpit specifically is involved.
The first and most important is the heart. Cardiac chest pain from angina or a heart attack classically radiates to the left arm, shoulder, or jaw, and it can be felt in the armpit region. That said, heart-related pain has some recognizable patterns: it tends to come on with physical exertion or emotional stress, feels like pressure or squeezing rather than a sharp stab, and does not change when you press on the area or shift your position. A review in JAMA found that chest pain that is stabbing, positional, or reproducible by pressing on it carries low likelihood ratios for acute coronary syndrome, while pain that radiates to the shoulders or arms or is triggered by exertion carries meaningfully higher ones.9JAMA Network. Value and Limitations of Chest Pain History in the Evaluation of Patients With Suspected Acute Coronary Syndromes In plain terms: sharp, localized pain that you can poke and make worse is much less likely to be your heart than a deep ache that shows up when you climb stairs.
The second, less common scenario involves the spleen. The spleen sits under the left rib cage, and when it is inflamed or injured, it can irritate the diaphragm and produce referred pain in the left shoulder area, a phenomenon called Kehr’s sign. A case report described a young woman whose chief complaint of left shoulder pain turned out to be caused by a splenic abscess.10PubMed. Traditional Kehr’s sign: Left shoulder pain related to splenic abscess Splenic issues are rare, but if your left-sided pain came on suddenly after abdominal trauma or alongside fever and abdominal tenderness, it is worth mentioning to a doctor.
Red Flags That Mean You Should Not Wait
Most causes of armpit pain resolve on their own or with simple treatment. There are situations, however, where waiting is the wrong move:
- Pressure-type pain with exertion: If the pain feels like squeezing or heaviness, radiates to the arm, jaw, or back, and worsens with physical activity, treat it as a possible cardiac event and seek emergency care immediately.
- Accompanying shortness of breath, dizziness, or nausea: These paired with left-sided pain strengthen the case for a cardiac evaluation.
- A hard, fixed, painless lump: Paradoxically, the lumps that hurt less tend to be more concerning. A hard, immovable node that has been growing over weeks warrants prompt imaging.
- Unexplained weight loss or night sweats: Combined with a persistently swollen node, these suggest a systemic process like lymphoma that needs investigation.
- Red streaks spreading from the armpit: This pattern suggests a spreading infection (cellulitis or lymphangitis) and may require antibiotics quickly.
None of these red flags mean the worst-case scenario is confirmed, but they do mean sitting at home and hoping it goes away is not the right strategy.
What Happens When You Get It Checked Out
If you visit a doctor for persistent armpit pain, the workup is usually straightforward. A physical exam covers a lot of ground: pressing on the muscles to check for tenderness, feeling the lymph nodes for size and texture, examining the skin for lumps or rashes, and testing your arm’s range of motion. Many causes become obvious at this stage.
Ultrasound is the go-to imaging tool for the armpit. It can distinguish between swollen lymph nodes, cysts, abscesses, and collections of fluid. One large review of nearly 20,000 diagnostic breast ultrasounds found that incidental axillary findings warranting follow-up occurred in only about 0.5 percent of exams, and none of those incidental findings turned out to be an occult breast cancer.11PubMed. To scan or not to scan: effect of scanning the axilla of all patients undergoing diagnostic breast ultrasound That statistic is reassuring: the overwhelming majority of things found incidentally under the arm are benign.
For breast-related concern specifically, mammography and ultrasound are often combined. A study of nearly 1,000 patients presenting with breast pain found that roughly 63 percent were evaluated with both mammography and ultrasound, while the remainder had one or the other depending on age and clinical picture.12PubMed. Breast pain and cancer: should we continue to work-up isolated breast pain? Isolated breast or armpit pain without a palpable lump and without worrisome imaging findings carries a very low probability of underlying cancer, which is why many doctors take a reassure-and-monitor approach in those cases.
After Breast or Axillary Surgery
If you have had surgery in the breast or armpit area, a distinct set of causes enters the picture. Axillary web syndrome, sometimes called “cording,” occurs when hardened tissue bands form after lymph node removal, creating visible cord-like structures that restrict shoulder movement and cause pain down the arm. One study found axillary web syndrome in about 28 percent of patients who underwent axillary surgery, with the risk substantially higher in those who had full axillary lymph node dissection compared to sentinel node biopsy alone.13PubMed. Incidence and risk factors for axillary web syndrome after breast cancer surgery Physical therapy with gentle stretching is the primary treatment, and the cords typically soften over weeks to months.
Another post-surgical cause that is easy to overlook is a trapped collection of blood, called a haematoma, forming in the armpit after node dissection. Researchers reported cases in which severe post-mastectomy pain syndrome was completely relieved by draining an encapsulated haematoma from the axilla, suggesting that pressure from even a small blood collection can irritate surrounding nerves.14PubMed. Some cases of severe post-mastectomy pain syndrome may be caused by an axillary haematoma Because these haematomas are not always obvious on physical exam, they can be missed if nobody thinks to look for them. If you are experiencing persistent armpit pain months after breast surgery that does not match the typical recovery timeline, asking about the possibility of a small fluid collection is worthwhile.
Why the Left Side Specifically
In most of the causes listed above, there is nothing special about left versus right. Muscle strains, lymph node reactions, skin conditions, and nerve irritation can all happen on either side. The reason people tend to worry more about the left underarm is its proximity to the heart. That concern is not irrational, but it is frequently misplaced. Cardiac pain has a recognizable character that is quite different from the sharp, localized, position-dependent pain that most armpit complaints produce.
There is one anatomical asymmetry worth noting. If you are right-handed, your left arm often does more stabilizing and less active movement, which can make the muscles on that side more susceptible to strain during unusual activities. Conversely, if you are left-handed, your dominant arm’s muscles may be more conditioned but also more heavily used, making overuse injuries slightly more likely on that side. Neither pattern is dramatic enough to explain a strong left-right preference for armpit pain in the population, but it can explain why a particular episode affects one side rather than the other for a given individual.
If your left-sided armpit pain is sharp and reproducible by pressing on it, started after physical activity or a new product, and is not accompanied by chest pressure, shortness of breath, or exertional symptoms, you are almost certainly dealing with one of the benign causes above. Keep an eye on it, try the obvious fixes, and see your doctor if it persists beyond a couple of weeks or changes character.