What are the causes of erythema nodosum (EN)?

Causes of erythema nodosum (EN)

• Bacterial infections: Streptococcus, Yersinia, Salmonella, Campylobacter, Chlamydia, Mycoplasma 
• Mycobacterial infection: TB, leprosy 
• Viral infection: Infectious mononucleosis, hepatitis B, herpes simplex 
• Other infection: Tinea, toxoplasma 
• Drugs: Sulfa drugs, OCP, potassium iodine agents 
• Malignant disease: Malignant lymphoma (Hodgkin's disease etc.), leukaemia 
• Inflammatory bowel disease: UC, Crohn's disease 
• Others: Sarcoidosis, Behcet's disease, reactive arthritis (Reiter's syndrome), pregnancy 

Sources:

• Shimizu's dermatology 2nd Edition; page:385 and table 18.3
• Rapid review of clinical medicine for MRCP part 2 by Sanjay Sharma (question no: 401)
• Dermatology made easy 1st Edition; pages:571 & 576