05/13/2015 05:32 EDT | Updated 05/13/2016 05:59 EDT

It Is Possible to Counteract (Some) Drug Allergies

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QUESTION: My doctor told me to take a baby Aspirin a day to protect my heart. But then I started getting stuffed-up sinuses and itchy skin. My doctor says I have developed an allergy to Aspirin. This seems strange. I've taken Aspirin before and not had a problem. My doctor now wants me to do a treatment called "desensitization." I will be given small doses of Aspirin, which will be gradually increased until I develop a tolerance to the drug. It sounds like voodoo. Does it really work?

ANSWER: Yes. Desensitization, which involves gradual reintroduction of a drug, starting with very small doses, makes it possible for some people to take a medication that would normally trigger an allergic reaction. The procedure, of course, must be done under direct supervision of a health-care professional in case you develop a severe reaction. It is not something you would attempt to do on your own.

You noted in your question that you didn't get allergy symptoms when you took Aspirin in the past. It is possible to develop a new allergy to a drug later in life. For whatever reason, it can happen.

Aspirin allergies can produce a wide variety of symptoms, ranging from mild to severe. Symptoms usually develop within an hour after taking the drug and may include a runny nose, red eyes, itchy skin, hives, coughing, wheezing or shortness of breath and, most serious of all, a potentially deadly narrowing of the airways -- commonly known as anaphylaxis.

An allergic reaction is the body's immune system responding to some substance that is perceived to be an invader or a threat. It is really an inappropriate response -- or an over-reaction -- to a common and otherwise harmless substance (such as pollen, certain foods and dust). Sometimes patients have reactions to drugs that mimic allergies, but which occur by other mechanisms. Depending on the drug and the type of reaction that occurs, desensitization may be appropriate.

It can be worthwhile for patients to go through the time-consuming desensitization process if they need Aspirin daily for a particular medical condition, says Dr. Elizabeth Weber, consultant and head of testing at the Drug Safety Clinic at Sunnybrook Health Sciences Centre.

In addition to being a pain reliever, Aspirin -- also known by the generic name of acetylsalicylic acid or ASA -- has an effect on clot-forming platelets in the blood. It makes them less sticky, so they are less likely to form clots that could result in a heart attack or stroke.

Aspirin can also help prevent blockages of stents -- devices that open up narrowed blood vessels.

That's why Aspirin is commonly recommended for patients who suffer from cardiovascular disease.

There are several different protocols for carrying out desensitization, but they follow the same general principle of incrementally increasing the dose.

"We start with small doses and work our way up and we sneak the drug into your body," explains Dr. David Juurlink, a drug-safety expert and head of Clinical Pharmacology and Toxicology at Sunnybrook.

"What we are essentially trying to do is trick the immune system into accepting the drug. And, more often than not, it succeeds."

At Sunnybrook, the procedure is performed on inpatient wards or in the Drug Safety Clinic. The hospital's pharmacy prepares all the doses in advance.

In the case of Aspirin allergies, a patient is started at a dose of one milligram, says Jackie Campbell, the drug-safety pharmacist at Sunnybrook.

"Throughout the protocol, we monitor your blood pressure, your heart rate, how you are breathing and we observe you for signs of a reaction," says Ms. Campbell.

If the patient remains symptom-free after half an hour, the dose is doubled. These steps are repeated until the patient reaches a final target dose, which is usually 81 milligrams -- the amount of medication in a baby Aspirin. The patient remains in the clinic for several more hours of observation.

Once the procedure is completed, the patient should be able to resume taking a daily dose of baby Aspirin without suffering ill effects.

But the patient must continue taking the drug at that same dose every day -- not missing a dose -- or the tolerance will be lost. "You would need to be re-desensitized and have to go through the protocol all over again," says Ms. Campbell.

Desensitization is by no means permanent. But it makes it possible for a patient to tolerate a drug for as long as it is needed.

Ms. Campbell points out that when patients develop allergies to particular drugs, there may be alternative medications they can take that work just as well. But there is not always a good alternative. So desensitization is routinely used for certain drugs that lack suitable substitutes.

And, when it comes to some cardiovascular conditions, a relatively inexpensive baby Aspirin is often the ideal choice.

Co-authored by Paul Taylor, Personal Health Navigator at Sunnybrook.


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