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Harvard Commentaries
Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School

Medical Myths Medical Myths

Four Common Myths About Allergies

October 10, 2013

By Robert H. Shmerling M.D.

Beth Israel Deaconess Medical Center

Healthy Lifestyle
Medical Myths
Four Common Myths About Allergies
Four Common Myths About Allergies
While allergies of one sort or another are common, so are misconceptions about allergies. Here are some of the most common.
Harvard Medical School Commentary

While allergies of one sort or another are common, so are misconceptions about allergies. Here are some of the most common.

1. "I can't take any antibiotics — I'm allergic to them all. "

Allergic reactions to medications are common, and antibiotics are among the biggest culprits for provoking allergic reactions. And if you are allergic to an antibiotic, it is common to "cross-react" with an agent that is chemically similar or a member of the same family. For example, if you are allergic to penicillin, there's a good chance you will also have a reaction to amoxicillin (a close relative) or even cephalexin (Keflex, a more distant relative) because parts of their chemical structure are similar.

However, it is unusual to be allergic to more than one or two types (or families) of antibiotics, because different types are chemically unrelated. For example, having an allergy to penicillin does not generally increase your chances of being allergic to sulfa-containing antibiotics (such as Bactrim). Knowing your specific allergy history is important because antibiotics can be very helpful — even life-saving — for bacterial infections, and you'd like to be able to take the most useful ones when needed.

Another source of confusion is separating out a non-allergic side effect of a medicine from a true allergy. Having an upset stomach while taking erythromycin is probably not an allergic reaction, but having a rash probably is. While it doesn't always matter — if you have a bad side effect or a bad allergy, you'd like to avoid the medicine in any event — the distinction between a side effect and a true allergy is sometimes important to make.

For example, if you have stomach upset with erythromycin, a closely related medicine, such as clarithromycin might be well-tolerated; but if you had trouble breathing after taking erythromycin, it might actually be dangerous to take clarithromycin. And in some situations, a doctor might recommend a medicine even if there have been side effects in the past (though rarely if a severe allergic reaction was experienced). The difference between a mild allergic reaction and a nonallergic side effect could determine the best way to get around the problem: An allergic reaction might be overcome with "desensitization," a technique in which a person takes tiny doses of the medication with a gradual increase in dose over time. This would be unlikely to work well if the problem was a side effect of the medication such as diarrhea; for that sort of nonallergic side effect, it might be worth tolerating the symptoms for a short time or treating it separately (for example, with loperamide, or Imodium).

If you're the one with the allergic reaction or other side effect, you'd like to avoid that medication in the future. For this reason, let your doctors know what medicines you have taken in the past that caused problems, but be sure to mention what the problem was so that true medication allergies can be distinguished from side effects.

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2. "I'm allergic to so many foods, I really have to watch what I eat. "

Food allergies do occur and are important to those who have serious reactions to specific foods. But the problem is not as common as you might think. Surveys suggest that 40% of adults report a food allergy, but when investigated, only about 1% to 2% of adults are truly allergic. A slightly higher rate is observed among children, but still only about 5% of the total population has significant food allergies.

The faulty assumption of food allergy probably relates to the failure to distinguish a nonallergic side effect from true allergy, just as described above for drug allergies. As an example, lactose intolerance is a remarkably common condition that makes people unable to digest lactose-containing foods, such as dairy products. If they do eat or drink these foods, gas, bloating, cramping and diarrhea may follow. They may mistakenly believe they are allergic, but the problem is actually due to an inherited deficiency in the ability to produce lactase, the enzyme needed to digest lactose.

Another reason that food allergies are reported more often than is the case is that two commonly occurring events (gastrointestinal symptoms and eating) are mistakenly linked to one another in an effort to explain the course of events. For example, if you eat an unusually rich dinner at a fancy restaurant and the next day you develop diarrhea, it may seem logical to blame one of the foods you ate. In fact, you may notice you are repulsed by the thought of eating that food again — the associations we make can be strong, even when made in error. In fact, the symptoms could just as easily have been caused by the lunch or snack you had before the fancy dinner, the midnight snack, or a coincidental viral infection. While a food allergy is always possible, it is not as common as many people assume.

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3. "We have to get a dog that doesn't shed because my sister is allergic. "

As illogical as it may seem, people who are allergic to animals are usually reacting to the skin or even saliva of the pet, not the fur. So, a short-haired pet or one that doesn't shed doesn't necessarily make a better housemate for the allergic person. Petting a short-haired dog can cause just as much trouble if you are unlucky enough to be allergic.

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4. "My allergies are awful these days. I'd better stay inside."

While pollen, grass, and other plant-borne allergens are common causes of seasonal allergies, there's also a reasonable chance that your home is the source of your allergies — and they may only get worse by staying indoors. For example, dust mites are common in bedding, mold tends to grow around the edges of windows, and many bathroom detergents and cleansers provoke allergic reactions after inhalation or with contact. Fortunately, there is much you can do to prevent these problems:

  • Use gloves when cleaning the bathroom and rinse cleaning agents thoroughly.
  • Use antifungal cleaners where mold tends to collect.
  • Wash all bedding in hot water regularly to kill dust mites. Other measures include use of chemical treatment of carpets and furniture, use of a dehumidifier and air conditioner, use of furniture made primarily of wood rather than stuffing, avoidance of wall-to-wall carpet and feather or down pillows.

These simple measures can reduce the chances that you'll be sneezing because of something in your home.

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The Bottom Line

As a first step in combating allergy, most experts in allergic disease will recommend avoiding exposure to whatever triggers your allergies. But, to do that you must identify those triggers and understand your condition. Taking medications for allergies will not help if your symptoms are due to something else. Likewise, avoiding pollen won't help if you are allergic to mold. If you can figure out what sets off your allergies and you can easily avoid it, you may not need to take any medications to improve the situation. If your allergies are a mystery to you and the simple remedies fail (such as avoidance or over-the-counter medications), it may be time to see your health-care provider or an allergy specialist. Even people who understand the difference between allergy myth and fact sometimes need help to breathe easily.

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Robert H. Shmerling, M.D., is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.

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