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Antihistamine Symptoms

Antihistamines are some of the most widely used drugs, found in countless over-the-counter (OTC) and prescription medications. Although their primary application is in the prevention and alleviation of allergic reactions, antihistamines also are used in OTC sleep aids and motion sickness pills. Although the shelves at your local pharmacy or grocery store may be overflowing with antihistamine-containing medications, it's important to remember that they are drugs and can cause unpleasant symptoms or side effects.

  1. How They Work

    • When your body's immune system detects an irritant or an allergen you've inhaled or perhaps has touched your skin, it releases histamines, a type of protein. The histamines travel to the area that first encountered the allergen, bind to the receptors of cells in that part of your body, and trigger a syndrome of symptoms that can be downright unpleasant. The most obvious of those symptoms is inflammation, which in your upper respiratory system might quickly progress to sneezing and/or a runny nose while, on your skin could cause an itchy rash. Antihistamines, particularly effective when taken before exposure to an allergen, coat the receptors, making it difficult, if not impossible, for histamines to trigger the symptoms associated with an allergic reaction.

    Symptoms Generally Mild

    • The symptoms or side effects of antihistamines generally are mild in nature and fall into a few broad categories: central nervous system (CNS) symptoms and those associated with your gastrointestinal (GI) system and upper respiratory system.

    CNS Symptoms

    • Many antihistamines, including virtually all of the so-called first generation of these drugs, cause drowsiness. This explains their use in OTC sleeping aids. Related symptoms or side effects can include mental confusion, dizziness, restlessness, excitability, irritability, anxiety and poor coordination. With the exception of drowsiness, these symptoms, if you experience them at all, usually pass quickly.

    GI and Upper Respiratory Symptoms

    • Like the side effects that target your central nervous system, those associated with your GI and upper respiratory systems usually are brief in duration, disappearing after your body adjusts to the drug. However, GI symptoms can include an upset stomach, nausea and loss of appetite. Common symptoms of the upper respiratory system include dryness of your nose and mouth. Your eyes, which are not part of either the GI or upper respiratory systems, also may be dry and experience brief periods of blurred vision.

    Serious Symptoms

    • While the symptoms of antihistamines generally are mild and no cause for alarm, you can, in rare cases, experience more serious symptoms. These include fever and/or chills, breathing difficulties, fainting, convulsions, hallucinations, a sharp drop in blood pressure, uncontrolled movements and extreme sensitivity to light. Anyone experiencing such symptoms should seek medical attention immediately.

    Who Should Avoid Antihistamines?

    • Unless your doctor specifically authorizes it, do not take antihistamines if you suffer from high blood pressure, heart disease, breathing difficulties, glaucoma, thyroid disease or prostate problems.

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