In about half of cases, the exact cause of erythema nodosum is unknown. The remaining cases are associated with an infection or other systemic disorder.
Some of the more common infections associated with the disorder are:
- Streptococcus (most common)
- Cat scratch disease
- Hepatitis B
- Mononucleosis (EBV)
Erythema nodosum may occur with sensitivity to certain medicines, including:
- Antibiotics, including amoxicillin and other penicillins
- Birth control pills
Sometimes, erythema nodosum may occur during pregnancy.
The condition is more common in women than it is in men.
Erythema nodosum is an inflammatory disorder. It involves tender, red bumps (nodules) under the skin.
Exams and Tests
Your health care provider can diagnose this condition by looking at your skin. Tests that may be done include:
- Punch biopsy of a nodule
- Throat culture to rule out a strep infection
- Chest x-ray to rule out sarcoidosis or tuberculosis
- Blood tests to look for infections or other disorders
Erythema nodosum is uncomfortable, but not dangerous in most cases.
Symptoms most often go away within about 6 weeks, but may return.
Erythema nodosum is most common on the front of the shins. It may also occur on other areas of the body such as buttocks, calves, ankles, thighs, and arms.
The lesions begin as flat, firm, hot, red, painful lumps that are about 1 inch (2.5 centimeters) across. Within a few days, they may become purplish in color. Over several weeks, the lumps fade to a brownish, flat patch.
Other symptoms may include:
- General ill feeling (malaise)
- Joint aches
- Skin redness, inflammation, or irritation
- Swelling of the leg or other affected area
The underlying infection, drug, or disease should be identified and treated.
Treatment may include:
- Nonsteroidal anti-inflammatory drugs (NSAIDs).
- Stronger anti-inflammatory medicines called corticosteroids, taken by mouth or given as a shot.
- Potassium iodide (SSKI) solution, most often given as drops added to orange juice.
- Other oral medicines that work on the body's immune system.
- Pain medicines (analgesics).
- Raising the sore area (elevation).
- Hot or cold compresses to help reduce discomfort.
When to Contact a Medical Professional
Call your provider if you develop symptoms of erythema nodosum.
Forrestel A, Rosenbach M. Erythema nodosum. In: Lebwohl MG, Heymann WR, Berth-Jones J, Coulson IH, eds. Treatment of Skin Disease: Comprehensive Therapeutic Strategies. 4th ed. Philadelphia, PA: Elsevier; 2018:chap 75.
Gehris RP. Dermatology. In: Zitelli BJ, McIntire SC, Norwalk AJ, eds. Atlas of Pediatric Diagnosis. 7th ed. Philadelphia, PA: Elsevier; 2018:chap 8.