| Idiopathic |
| Familial |
| Pregnancy (particularly second trimester) |
| Systemic diseases |
| Inflammatory bowel disease |
| Sarcoidosis |
| Malignancy |
| Collagen vascular disease |
| Sweet's syndrome |
| Drugs |
| Antibiotics (penicillin, sulfonamides) |
| Birth control pills |
| Omeprazole |
| Immunizations |
| Infections |
| Streptococcal pharyngitis |
| Tuberculosis |
| Salmonella gastroenteritis |
| Cat scratch fever |
| Leprosy |
| Yersinia colitis |
| Psittacosis |
| Histoplasmosis |
| Blastomycosis |
| Coccidiomycosis |
