What are the causes of erythema multiforme?

Causes of erythema multiforme:

• Infections (at least 90% of cases):
• Virus: Mostly by human herpes simplex virus.  But Many different virus infections have been reported to trigger erythema multiforme including: 
• Parapoxvirus (orf and milkers’ nodules)
• Herpes varicella-zoster (chickenpox, shingles)
• Adenovirus
• Hepatitis viruses
• Human immunodeficiency virus (HIV)
• Cytomegalovirus
• Viral vaccines
• Bacteria (Streptococcus, Mycoplasma, non‐tuberculous mycobacteria)
• Tinea
• Chlamydia
• Rickettsia
• Drugs:
• Antibiotics (Penicillin, sulphonamides)
• NSAID
• Anticonvulsants (Barbiturates)
• Phenothiazine
• Antineoplastic agents
• Allergic disorder:
• Insect bite
• Collage disease:
• SLE
• Other disease conditions:
• Sarcoidosis
• Crohn’s disease
• Hematopoietic malignant disorders
• Others:
• Physical stimulant (e.g. cold)
• Pregnancy

Sources:

• Shimizu's Dermatology 2nd Edition; Table 9.1, page: 146
• Rapid Review of Clinical Medicine for MRCP Part 2 Second Edition by Sanjay Sharma: Q 154
• Dermatology Made Easy 1st Edition (2017): Chapter 3; Section 3.7