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What Is a Second-Generation Antihistamine?

By S. Berger
Updated May 17, 2024
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Allergies can lead to a variety of symptoms like itchy eyes, runny nose, and hives, and they are caused by overactivity of a naturally produced compound called histamine. Antihistamine medications reduce the efficacy of this compound to offer relief of these symptoms. First-generation antihistamines, the initial varieties of these medications to be discovered, often cross into the brain, creating side effects like dizziness, confusion, and sleepiness; another class, known as the second-generation antihistamine drugs, were developed to help avoid these adverse reactions.

Second-generation antihistamine compounds are distinctive because they restrict their actions to the body, and do not cross into the brain in any large volumes. Consequently, they lack some of the side effects that other antihistamines have, particularly drowsiness, and are sometimes referred to as "non-drowsy antihistamines." The cause of this specificity for the body is because these molecules are highly polar, preventing them from going through the non-polar blood brain barrier that screens some compounds from entering the brain.

In many countries, second-generation antihistamine drugs are generally available by prescription, but some of these compounds, including loratidine, may be purchased over the counter. Regardless of their legal status, however, the uses of antihistamines in this class are similar. Often, they are used for treating allergies in those who do not respond to treatments involving first-generation antihistamines. Sometimes, they may be used in individuals that experience severe side effects from older antihistamines, as well. Usually, these medications take up to an hour to be effective after ingestion, which means they provide the greatest relief when they are administered before exposure to allergens.

The second-generation antihistamine drugs are not without the potential for side effects of their own. Some individuals are faced with the chance of experiencing heart issues, known as cardiac effects. In rare cases, these cardiac issues may even be lethal. Second-generation antihistamine medications can demonstrate an increased risk for these dangerous effects when they are taken at high doses. More often, however, the most severe effects that occur are headaches, dry mouth, and cramps.

Despite the fact that the second-generation antihistamine family does not cause sleepiness in most individuals, this effect and others, like dizziness, may sometimes occur. Taking large doses of these second-generation drugs may cause a larger amount of the medication to enter the brain, and extreme sleepiness can result. Occasionally, mixing first- and second-generation antihistamines may also produce a similar effect. Most individuals using these medications must therefore use caution when combining them, so as to avoid unnecessary interactions.

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