Dear Dr. Roach: About 45 years ago, I came down with a rash on my chest and stomach from taking ampicillin. I was told never to take penicillin again. I’m allergic to many antibiotics, which makes treating me (an elderly person) very challenging.

Now my doctor feels that I should be able to take ampicillin. The pharmacist would not fill out the prescription because she felt that the antibiotic could trigger an allergic response, as this is what happened to her.

My sister and father were allergic to penicillin, so I wonder if this tendency is inherited. Supposedly, research shows a change of mind with penicillin allergies, and a past rash is no longer an indicator of an allergy. Please share your thoughts and knowledge. — D.W.

Answer: It isn’t always easy to tell whether a rash from an antibiotic, particularly a penicillin-type antibiotic, is a serious allergic reaction. A rash is a common occurrence when an antibiotic like ampicillin or amoxicillin is given to a person with a virus, especially infectious mononucleosis. It can look very much like an allergic response, but it isn’t a true allergy.

It is safe to give penicillin-type drugs to people with this kind of reaction. Many people are labeled as allergic to penicillin, causing a useful class of antibiotics to be avoided for no good reason. A family history of a penicillin allergy isn’t particularly important.

When a person has had a suspected allergic reaction, and there is involvement of the mucous membranes, or there is a new fever that begins with the rash, then it is highly suspicious for a severe reaction. So, a careful history of the initial rash is important in deciding whether it is appropriate to use the drug that is suspected of causing the allergy. It may be tough to remember the details from 45 years ago.

Many times, it is fine to never try using the antibiotic again. We have many different classes of antibiotics, and it may not be critical to use penicillin again, even if it isn’t a true allergy. But there are a handful of infections that require penicillin drugs. If the decision is made to try giving the antibiotic, it should be an explicit decision made and performed when resources are available to handle a serious allergic reaction if it occurs. I wouldn’t just prescribe it and hope for the best.

Penicillin skin testing is well-described, but few physicians besides allergists know how to do or interpret them. Desensitization to penicillin can also be done, although this is not a permanent cure for a penicillin allergy.

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.