Three weeks ago I had surgery and had a blood transfusion following the surgery. I have taken Penicillin many times before and have never had a problem. In the hospital, I was on IV Ancef (Cephalosporin, similar to Penicillin) and did not have a problem. Now, 3 weeks after the surgery, my doctor prescribed Augmentin, which is a broad-spectrum penicillin antibiotic, and I broke out in a rash over my entire abdomen, chest and thighs on the 2nd day that I was taking the medication. I haven't changed any products, foods, or other medications, so I am fairly certain it is a penicillin allergy. Does anyone know if you can develop new medication allergies from a blood transfusion? Thanks!