Lauran Neergaard, AP

Having a bad reaction to penicillin as a child doesn't guarantee you're still allergic decades later. And if the oncologist says you have to switch chemotherapies because of an allergic reaction, well, maybe not.

More medical centers are recommending a lesser known choice: Drug desensitization, a carefully controlled method of helping patients temporarily tolerate medications that their bodies once rejected.

Not everyone's a candidate. But for those who are, the process can mean the difference between getting the best treatment or a runner-up that may not do the job, says Dr. Mariana Castells, an allergist at Harvard and Brigham and Women's Hospital who helped pioneer the care. “You don't know how lucky I feel to have been desensitized,” says Vanessa Greenleaf of Marblehead, MA.

Greenleaf developed a severe allergy to a mainstay of ovarian cancer treatment, carboplatin. Even as a burning sensation engulfed her body during the allergic attack, Greenleaf's chief fear was that doctors at Massachusetts General Hospital would take her off the chemo combination that she believed was her best shot.

Allergies make up five to 10 percent of all adverse reactions to medications, according to the American Academy of Allergy, Asthma and Immunology. Sometimes drug allergies kill. So anyone who's ever reported an allergic reaction to a medicine, even decades earlier, is told never to take that drug.

Penicillin and related antibiotics are a leading trigger, as are anti-inflammatory painkillers like aspirin. But increasingly, severe reactions to some leading cancer therapies are being reported, including allergies developed by more than a quarter of patients who take widely used platinum-based chemos.

You've probably heard of allergy shots, where hay-fever sufferers get small doses of the problem allergen for a few years until they build up tolerance to it. More recently, allergists have begun testing similar therapy for children with life-threatening food allergies.

Desensitization for drug allergies is the same concept. It was first started for penicillin allergies and expanded to chemo in recent years. Tiny, diluted amounts of the problem drug, sometimes with anti-allergy medicines, are given in slowly escalating doses over hours to a day — usually in the hospital or even intensive care unit for safety — until reaching the optimal dose.

But desensitization's effects last only as long as the patient is taking a daily dose of that medication. That means chemo patients, for instance, must get desensitized all over again before each new treatment cycle, but the culprit isn't always obvious.

One man had his open-heart surgery aborted after he had a reaction to either the anesthesia or a precautionary antibiotic. His surgery can't be rescheduled until the cause is found. When patients don't have a good alternative, customized desensitization can calm the immune cells, called mast cells, that control this allergic response. Small amounts of the drug can bind to the cells in a way that prevents them from overreacting to the bigger dose.