How likely is an anaphylactic reaction to a new antibiotic on first round? would i normally get a rash first? have a prescription to fill and nervous.

Very low for most: In order to have an allergist reaction you have to have "seen" the substance before. This can occur by seeing a similar antibiotic, or after you have had several doses of a new antibiotic. Most people don't have anaphylactic reactions to antibiotics. Assuming you have not had anaphylaxis to another antibiotic, fill the prescription and don't worry.