While the rash can be painful, it is harmless in itself. It usually settles down without specific treatment over several weeks.

Erythema nodosum causes

Erythema nodosum happens when the layer of fat that everybody has under their skin becomes inflamed or irritated.

It can be triggered by:

• a reaction to medicines such as the contraceptive pill or a sulfur-containing medication
• a throat infection
• pregnancy

It can also come on in people who have:

• inflammatory bowel disease such as Crohn's disease and ulcerative colitis
• tuberculosis and other infections
• sarcoidosis

But in about half of all people who get it, doctors can’t identify the cause.

Erythema nodosum symptoms

Painful red lumps or nodules gradually appear on the skin over a period of up to 10 days. They usually appear on the legs, from the knees down. They can also appear on the thighs, arms or face. The lumps can be as small as a grape or as big as an orange. They gradually turn purple, like an old bruise, before fading away.

Other symptoms can include:

• swollen legs
• fever
• feeling unwell
• aching joints

Erythema nodosum diagnosis

The rash is diagnosed by a doctor based on how the bumps look and feel. Sometimes a biopsy (sample) of your skin is taken to make sure the diagnosis is right. If your doctor suspects that there is some other illness causing your erythema nodosum, they might ask you to have other tests.

Your doctor might order:

• blood tests
• throat swab to test for streptococcal infection
• chest X-ray for tuberculosis
• phlegm test
• urine test
• stool test

Erythema nodosum treatments

Your doctor will treat any illness that is causing erythema nodosum.

General treatment for erythema nodosum might include:

• anti-inflammatory medicine (some people can't take these - check with your doctor if unsure)
• support stockings or bandages
• resting, especially if your legs are sore and swollen
• raising your feet and legs
• cold packs

Most people find their erythema nodosum clears up in 2 to 4 weeks, without any need for medicine.

If you think you might have erythema nodosum, it is best to seek advice from your doctor.