In order for your boyfriend to be having an allergic reaction, he would have to be taking the medication. I am not aware of any cases where Amoxicillin would be in the vaginal secretions. If so, the amount would be very insignificant. If his rash continues, he should be seen by his family doctor. In the meantime, until you know if the rash is communicable, if may be wise to avoid intimate contact.
19. February 2012
anonymous-qa