You've just taken a new medication. Within minutes, you're having trouble breathing and your vision is blurry. You might be experiencing a drug allergy — a sometimes-serious event caused not by a medication's therapeutic action or side effects, but by your immune system's misdirected attack on the medication.

True drug allergies — as opposed to nonallergic adverse reactions — account for fewer than 10 percent of all the drug reactions that land people in the hospital or develop during hospital stays. But because drug allergies may be severe enough to require specific anti-allergy treatment, it's important to distinguish them from other drug reactions. Before you take a new medication or a drug you haven't used in some time, ask your doctor if you could be allergic to it. That goes double if you've had a previous reaction to any drug.

A drug allergy is a specific type of reaction involving the same immune-system cells and chemicals that cause hay fever, peanut allergy and allergic contact dermatitis.

Typically, you won't have an allergic reaction to a drug when you first take it. Instead, the first time or first several times you take the allergy-causing drug, your immune system becomes sensitized, or primed to react when you next encounter that drug. After being sensitized, your immune system can quickly detect the drug in your body and churn out immunoglobulin E (Ig E) antibodies tailor-made to recognize the drug. These antibodies trigger the release of inflammatory chemicals, such as histamine, which cause allergy symptoms.

Sometimes an initial dose is all it takes to sensitize your immune system. Or you may take an entire 10-day course of a drug without any problem, only to break out in a rash and have your face swell up when you take the drug again a year later.  Also, if you're sensitized to one drug you might develop a drug reaction to a related drug, even taking it for the first time.

Most allergic and nonallergic drug reactions are mild, but either type can occasionally be severe. And while all drug allergies stem from an immune-system response, nonallergic drug reactions have a wide variety of causes, including the drug's intended mechanism of action. Every drug may cause side effects. A drug you take to relieve one symptom or condition may inadvertently cause new symptoms in different parts of the body.

This table details some of the differences between drug allergies and other adverse drug reactions:

The most common signs of drug allergies are skin reactions ranging from mild rashes to flaming red patches of hives. More severe reactions cause facial swelling, shortness of breath or dizziness. A drug may, rarely, cause anaphylaxis, an extremely severe allergic reaction in which blood pressure may fall rapidly and the throat may rapidly swell shut, cutting off the airway.

Nonallergic drug reactions may mimic true drug allergies, as is the case with aspirin-induced asthma. Aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen (Advil, Motrin, others), cause asthma symptoms, nasal congestion, watery eyes and, sometimes, facial flushing and swelling in about 10 percent of people who have asthma. It certainly sounds like a drug allergy involving the IgE immune system, but, in fact, it isn't.

Sensitization and IgE production play no part in aspirin-sensitive asthma. Instead, some people with asthma, often in combination with nasal polyps (harmless growths in the nasal lining), are vulnerable to asthma as a side effect of aspirin and aspirin-like drugs. In the body, these drugs inhibit an enzyme that produces inflammation and fever. Their ability to inhibit this enzyme — cyclooxygenase-1 (COX-1) — gives NSAIDs their therapeutic value as reducers of pain, inflammation and fever.

By inhibiting COX-1, however, NSAIDs clear the way for different enzymes that have undesirable effects in some people. One of these enzymes causes the release of chemicals that can inflame and narrow the airways and increase mucus production, causing an asthma attack. The process is a byproduct of the way NSAIDs work, not the result of an immune-system reaction to NSAIDs.

If you have a history of possible allergy to penicillin, you can have a skin test to find out for certain. Tests for other drug allergies are generally not as reliable as allergy testing for penicillin allergy.

If you develop allergy symptoms shortly after taking medication, see your doctor. You can take an antihistamine such as diphenhydramine (Benadryl, others) to relieve itching and calm a rash. More severe reactions may require treatment with oral or injected corticosteroids. Anaphylaxis is an emergency requiring an immediate epinephrine injection and hospital care to maintain blood pressure and support breathing.

Once you know you have a drug allergy, avoid the responsible drug and related agents. Tell all your health care providers, including your dentist, that you are allergic to the drug. In case you're in an accident, wear a medical alert I.D. bracelet so that emergency workers will know about your allergy.