Erythema nodosum is caused by an overreaction of the immune system to a trigger, most commonly an infection, medication, or underlying inflammatory disease. In many cases, though, no cause is ever found. Studies show that between 32% and 74% of cases are classified as idiopathic, meaning the trigger remains unidentified despite testing.
The condition produces painful, red, tender nodules, usually on the shins, and it develops when immune complexes deposit in the small blood vessels within the fat layer just beneath the skin. Understanding the known triggers can help you and your doctor narrow down what set off the reaction.
How the Immune System Creates the Nodules
Erythema nodosum is not an infection of the skin itself. It is a hypersensitivity reaction, meaning the immune system overreacts to something elsewhere in the body and the skin pays the price. The inflammation targets a very specific location: the walls (septa) that separate fat lobules in the tissue just below the skin’s surface. This is why the nodules feel deep and firm rather than surface-level like a rash.
The leading explanation is that immune complexes, small clusters of antibodies bound to antigens, get deposited in tiny blood vessels running through those fat walls. This triggers a delayed immune response that produces inflammation, free radicals, and signaling molecules that recruit more immune cells to the area. The result is swollen, tender lumps that can range from the size of a dime to a golf ball. They typically appear on the shins because gravity pulls immune complexes toward the lower legs, where blood flow is slower and vessels are more vulnerable to deposits.
Infections: The Most Common Trigger
When a cause is found, infection is the most frequent one. Streptococcal throat infections (strep throat) top the list and are the single most commonly identified trigger worldwide. The nodules typically appear one to three weeks after the throat infection, sometimes after the sore throat itself has already cleared. This delay fits the pattern of a delayed hypersensitivity reaction: the immune system needs time to mount the overblown response.
Other bacterial infections linked to erythema nodosum include tuberculosis, which remains an important trigger in parts of the world where TB is common, and certain gastrointestinal infections caused by bacteria like Yersinia and Salmonella.
Fungal infections also play a role, particularly in specific geographic regions. Coccidioidomycosis (valley fever), common in the southwestern United States and Mexico, and histoplasmosis, found in the Ohio and Mississippi River valleys, have both been linked to outbreaks of erythema nodosum. The CDC has documented clusters of cases tied to these fungal infections in affected communities. If you live in or have recently traveled to these areas and develop shin nodules alongside flu-like symptoms, a fungal infection is worth investigating.
Medications That Can Set It Off
Several classes of drugs are known to trigger erythema nodosum. Oral contraceptives are among the most commonly reported culprits, likely because of their effect on estrogen levels. Sulfonamide antibiotics (a class of antibacterial drugs) are another well-established trigger. Other medications that have been associated with the condition include certain antibiotics like penicillin and amoxicillin, as well as some anti-seizure medications.
Drug-triggered cases typically develop within the first few weeks of starting a new medication. If you notice tender nodules on your shins after beginning a new prescription, the timing can be a strong clue. Symptoms usually resolve once the medication is stopped, though it can take several weeks for the nodules to fully fade.
Inflammatory Bowel Disease
Erythema nodosum is the most common skin manifestation of inflammatory bowel disease (IBD). It affects 3% to 10% of people with ulcerative colitis and 4% to 15% of those with Crohn’s disease, with higher rates in patients whose disease involves the colon. In IBD, the nodules tend to flare alongside gut symptoms, so worsening bowel problems and new shin lumps appearing together is a recognizable pattern.
For some people, erythema nodosum is actually the first sign that draws attention to undiagnosed IBD. If you develop the nodules along with persistent diarrhea, abdominal pain, or unexplained weight loss, an evaluation for bowel disease is a reasonable next step.
Sarcoidosis and Löfgren Syndrome
Roughly 10% to 22% of erythema nodosum cases are attributable to sarcoidosis, an inflammatory disease that causes clusters of immune cells (granulomas) to form in organs, most often the lungs. When erythema nodosum appears alongside bilateral hilar lymphadenopathy (swollen lymph nodes in both sides of the chest, visible on X-ray), fever, and joint pain that moves between joints, the combination is called Löfgren syndrome. This specific pattern is so distinctive that it can be diagnosed without a biopsy.
One important detail: the erythema nodosum nodules themselves do not contain the granulomas characteristic of sarcoidosis. If a biopsy is needed to confirm sarcoidosis, it has to come from another site, not from the shin nodules.
Pregnancy and Hormonal Shifts
Pregnancy is a recognized trigger, though the exact mechanism is not fully understood. The prevailing theory is that the hormonal shifts of pregnancy, particularly rising levels of estrogen and progesterone, create a favorable environment for the immune overreaction that causes erythema nodosum. Immune complexes may form more readily during pregnancy, or the immune system may become hypersensitive to the hormones themselves.
This hormonal connection also helps explain why erythema nodosum is more common in women overall and why oral contraceptives are a known trigger. Women between the ages of 18 and 34 have the highest rates of the condition.
Less Common Causes
A number of other conditions have been linked to erythema nodosum, though each accounts for a small fraction of cases:
- Other autoimmune diseases: Behçet disease, lupus, and reactive arthritis can all produce erythema nodosum as a secondary feature.
- Cancers: Lymphoma and leukemia are occasionally associated with erythema nodosum, though this is rare. Persistent or recurrent nodules that don’t respond to treatment sometimes prompt screening.
- Other infections: Chlamydia pneumoniae, hepatitis B and C, Epstein-Barr virus, and HIV have all been reported as triggers in individual cases or small series.
How the Cause Is Identified
Because the list of possible triggers is long, finding the cause usually involves a stepwise approach. A throat swab or blood test for streptococcal antibodies is standard, since strep is the most common identifiable cause. A chest X-ray checks for the swollen lymph nodes seen in sarcoidosis and can also reveal signs of tuberculosis or fungal lung infections. Basic blood work looking for markers of inflammation, along with a review of your current medications and recent illnesses, rounds out the initial evaluation.
If those first-line tests come back normal and you don’t have symptoms pointing toward a specific condition, the diagnosis often lands on idiopathic erythema nodosum. That can feel unsatisfying, but it’s worth knowing that idiopathic cases tend to resolve on their own within three to six weeks and carry a good prognosis. The nodules fade through color changes similar to a bruise, shifting from red to purple to yellow-brown, and they do not leave scars or break open.
Recurrent episodes, on the other hand, warrant a deeper look. Repeated flares are more likely to have an underlying systemic cause like sarcoidosis, IBD, or a chronic infection that hasn’t been fully treated.