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Drug Allergy or Side Effect: What You Need to Know


About an hour after taking a new medication, you experience gastrointestinal distress. Could it be a side effect? It's common for people to try to self-diagnose an unwanted reaction to a medication. Such symptoms should be taken seriously because they could be side effects or allergic reactions.

If it is an allergic reaction, the medication must be stopped immediately. An allergic reaction such as anaphylaxis can cause serious respiratory symptoms, as well as fainting, itching and shock. If this occurs, you must stop using the medication and seek medical help immediately.

If you're experiencing a side effect, you may be able to deal with or minimize the effects and still reap the benefits of the medication.

Unwanted reactions from drugs are estimated to affect 10 percent of the world's population, and as many as 20 percent of these patients are hospitalized as a result.

What's a Side Effect?
Side effects—which are more common than drug allergies—are often caused by the way a medication works. For example, if a drug lowers blood pressure by relaxing the blood vessels, it could also cause dizziness because, when blood vessels are more relaxed, it may take longer for the blood to reach the brain when you stand from a seated position.

Other common side effects of medicine include vomiting, insomnia or sleepiness, nausea, joint pain and diarrhea, as well as constipation or heart palpitations. Some are more debilitating than others.

Side effects are often confused with or mislabeled as allergies, which can be much more harmful and potentially life-threatening.

How Is an Allergy Different?

An allergic response occurs when your body comes into contact with a substance it sees as foreign. To get rid of it, your body recruits the help of cells in the immune system that launch a response. Some allergic reactions may occur instantly, while others may take longer to develop.

Like seasonal allergies, allergic reactions to a medication can include swelling of the lips, tongue, face or respiratory system; itching; wheezing; or a rash or hives. Allergic reactions account for 5 percent to 10 percent of adverse drug reactions, according to the American Academy of Allergy, Asthma and Immunology. We are usually not born with drug allergies; they develop over time. If you take a medicine for a long time, your body may begin to produce antibodies to the drug that trigger a reaction after months or years.

For instance, Bobbie from Queens, NY, was diagnosed as a young adult with a penicillin allergy, one of the most common drug allergies, affecting an estimated 5 percent to 10 percent of people. She suffered from occasional urinary tract infections (UTIs), which couldn't be treated with penicillin, the usual drug of choice.

Instead, her health care providers prescribed Bactrim, which contains the antibiotics sulfamethoxazole and trimethoprim. In her 70s—after years of taking Bactrim—Bobbi developed an allergy to it that made her palms itch. Her health care provider prescribed another drug and told her she should avoid sulfa-based drugs.

To get a head's up on the side effects of medicine you are taking or are plan to take, ask your health care provider or pharmacist about potential side effects. You can also look up drugs on, a nonprofit website with an extensive database of popular medicines and their side effects, risks and benefits.

When you pick up a prescription, ask your pharmacist about your new medication and whether it could worsen effects of drugs you're already taking. Also, read the medicine bottle label as well as any accompanying material.

Over-the-counter drugs are required to include drug information on the back labels. If you don't find all of the answers there, you can usually find a toll-free number to call for additional details.

Remember that side effects can be greater among special populations, such as people aged 65 and older, as well as pregnant women and children.

The more you know about the differences between side effects and allergies, the more empowered you will be to make the right choices when it comes to medicine.

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