Is herpes simplex related to erythema multiforme?

Yes, sometimes: Erythema multiforme minor is an allergic reaction that looks like a bad case of hives but the bumps don't go away for days. Em minor is not serious. It is usually due to medicine allergies, such as penicillins or sulfa drugs. It can also be due to a reaction to an infection such as herpes simplex. However, em major (stevens johnson rash) is serious and usually due to drug allergies, not infection.

Yes: Often caused by allergic or hypersensitivity reactions to medications, but also can be caused b infections like herpes simplex virus.