A physician is likely to first investigate whether or not an individual’s symptoms are the result of a drug’s side effect or of an actual allergic reaction. The physician might inquire about a patient’s medical history and ask for a complete list of medications taken.

Patients should also be prepared to provide their physician with the full details of the reaction, including when the drug was ingested, what symptoms appeared, when symptoms began to appear and how long the symptoms lasted. The physician will then perform a physical examination to determine if any symptoms of an allergic reaction are still present.

An allergy skin test might be recommended if an allergy to penicillin is suspected. During this procedure, a tiny dose of penicillin is introduced to the skin via a scratch/prick or injection. If the skin reacts with a wheal or a raised red bump, it indicates an allergic response to the medication.

It should be noted that there is no licensed penicillin skin test in the United States. Physicians who perform skin testing to penicillin make up their own skin testing materials, and this may not be as accurate as a standardized test. 

In addition, a physician may worry about the risk of a serious allergic reaction to the testing and rule out the procedure as a result. In such cases, the physician will simply prohibit the patient from using the suspected drug.

In rare cases, a RAST (radioallergosorbent test) may be used on people who are too sensitive for a skin test. This type of blood test allows a laboratory to directly test a blood sample from an individual in an attempt to detect antibodies that correspond to a penicillin allergy.