A bullseye rash may be because of Lyme disease. Lyme disease can be treated effectively with antibiotics. In the initial stages single course of antibiotics will be sufficient.
There is no particular treatment for bullseye rash and only preventive measures can be taken The first preventive measure is to keep away from areas where ticks are prevalent.
In order to know the best treatment for a bullseye rash, you first need to know the reason for the rash in the first place. Once you know the cause, you can treat it.
