Severe allergic reactions to penicillin can be dangerous and life-threatening. You may be more likely to have this type of reaction if you have had:
- A positive skin test for penicillin allergy.
- Hives that appeared quickly after you took the penicillin.
- A previous anaphylactic reaction to penicillin.
If any of these apply to you, you should receive another antibiotic or undergo desensitization therapy. In this type of therapy, you start taking small amounts of the penicillin and gradually increase how much you take. This lets your immune system "get used to" the medicine, and you may no longer have an allergic reaction. Desensitization may have to be repeated if you have to use the antibiotic again in the future (desensitization doesn't last long).
You are not likely to develop an anaphylactic reaction to penicillin if you have had a rash that looks like measles that appeared from a few hours to days after you took penicillin.
