Recognizing a drug allergy for what it really is can be easy ... or difficult. It's easy if:

• You show symptoms while you take the drug of a few days after you stop;
• Symptoms disappear a few days after drug is discontinued;
• Your symptoms resemble those experienced by others who are allergic to the drug; or
• You’ve reacted to the same drug before in the same way.

It can be difficult to diagnose drug allergy, however, if:

• You have previously taken the drug in question with no difficult;
• Your symptoms resemble the symptoms of your disease (for instance, drug induced wheezing and bronchospasms may resemble respiratory problems from asthma);
• You are taking several drugs at the same time;
• The reaction persists for weeks of months after you stop taking the drug, as it the case with drugs that the body eliminates slowly, like Depo-Testosterone and other time release hormones; or
• You’re unknowingly exposed to unsuspected source of the drug (for example, penicillin in milk, or aspirin in over the counter remedies).

Overcoming these difficulties with skin tests to uncover the allergy is not really an option. For one thing, skin testing for drugs is risky because of the danger of anaphylaxis. For another, they don't offer accurate clues (except for the skin test for penicillin we mentioned earlier).

The biggest ”difficulty” is the first on our list: drug reactions that get lost among the symptoms of a basic, underlying illness. For instance, say you’re given streptomycin as a symptoms of the virus, when in fact it is a reaction to streptomycin. And that could be dangerous.

”A minor drug reaction may progress, if unrecognized, to more severe, even fatal, reactions,” writes allergist Richard D. DeSwarte, M.D., an assistant professor at Northwest University Medical School in Chicago.

Common sense, then, says that if you’re taking drugs of any kind you should immediately report unusual reactions to your doctor, even if you stopped taking the medication a few days before. That’s not only for your personal benefit, but it also helps doctors to keep track of reactions so that drugs can be used more safely and wisely.

For mild drug reactions, stopping the drug is all that’s usually needed. For widespread skin eruptions, tepid colloidal baths (described in detail in Contact (Skin) Allergies) can soothe and relieve the itch. For systemic reactions, like asthma, your doctor may prescribe a short course of antihistamines or corticosteroids to get things under control.

By staying away from a drug for a few years, many people lose their allergic sensitivity to it and are once again able to take it if they have to. Because immediate reactions are more dangerous than the gradual, delayed onset of symptoms, people with drug allergies (or drug allergies in the family) should read What to Do in an Allergic Emergency.