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By Mayo Clinic staffThe best way to avoid an allergic reaction to penicillin is to avoid penicillin and similar antibiotics altogether. If you're allergic to penicillin, it doesn't mean you're necessarily allergic to all antibiotics. Your doctor may be able to give you an antibiotic that's distantly related to penicillin without causing any problems. A number of antibiotics completely unrelated to penicillin also are available.
For some infections, it may be absolutely necessary for you to take penicillin. In these cases, and if you previously experienced a reaction to penicillin, an allergy skin test may be important. If the skin test reveals that you're sensitive to penicillin, your doctor may recommend desensitization.
During the desensitization process, you receive small but gradually increasing doses of penicillin orally or intravenously. Because desensitization can trigger an allergic reaction, it's attempted only in a controlled setting, usually a hospital — and only when penicillin is absolutely necessary. Your desensitization lasts only as long as you continue taking penicillin. If you stop and then need to take penicillin again later, you'll need to go through the desensitization process again.
The process of desensitization generally takes one to two days. However, it doesn't work in all cases.