What are antihistamines?

Quick Answer
Over-the-counter and prescription drugs that are commonly prescribed for allergy symptoms in order to reduce the inflammatory effects of histamines
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Indications and Procedures

Antihistamines provide a rapid form of treatment for controlling histamine-based physical reactions, including cold and allergy symptoms, rashes and hives, and insect bites and stings. They are also especially helpful for the allergic condition known as hay fever, which is related to seasonal allergies and allergic rhinitis. Antihistamines serve largely to decrease the discomfort and symptoms caused by histamine reactions. Symptoms of histamine reactions may include sneezing, itchy skin, rashes, a swollen throat, watery or itchy eyes, and a runny nose. In reducing these effects, antihistamines may also be useful in helping to decrease the risk of additional infections caused by the swelling of sensitive mucous membranes in the body, particularly in the sinuses, nose, and throat. Typically, if such membranes remain swollen, the passageways of the body, such as the sinuses, can become congested, making it easier for infections to develop.

Antihistamines are typically administered orally, in pill form, one or more times per day, but they are also available in the form of eye drops, nose sprays, and liquids. They are also combined frequently with other drugs such as decongestants to ameliorate further any uncomfortable cold symptoms and to prevent consequent problems such as sinus infections. Antihistamines alone are generally considered to be ineffective at reducing nasal congestion.

Uses and Complications

In addition to reducing histamine reactions, antihistamines have a number of helpful side effects, including mild reductions in nausea and motion sickness, a general effect of drying secretions, and an ability to induce drowsiness, which can help individuals initiate and maintain sleep. This ability to induce drowsiness can be helpful, for both adults and children who need a mild, nonaddictive sleep medication. It should be noted, however, that mild tolerance to the sedating effects of antihistamines has been observed and so use for such a purpose should be time limited.

Furthermore, any drug, including antihistamines, can be abused if used in larger quantities than prescribed or recommended. For instance, some individuals may use high quantities of antihistamines to experience a more pronounced sedating effect or other psychoactive effects. This is dangerous, as use of any substance outside its recommended use, particularly in situations requiring safety (for instance, driving, operating machinery, being around strangers, being in strange places) may lead to unexpected negative consequences.

Of the side effects mentioned, drowsiness persists in being the most problematic, as people needing to take the drugs more than once a day may have their daytime activities interrupted by feeling sleepy. Additionally, if antihistamines are combined with other substances, both prescription drugs or others such as alcohol, then drug synergies can occur. With sedating drugs such as alcohol, for instance, the onset of drowsiness can hasten and its strength can be greater than might be expected with either the antihistamine or the alcohol alone. As such, the use of antihistamines, particularly in combination with other sedating drugs (opioids, barbiturates, anesthetics, benzodiazepines, sedating pain medications), or even sedating alternative medicines, is not advised in situations requiring attention and alertness, such as driving or operating machinery.

In response to these problems, newer antihistamines have been developed to cause less drowsiness. These newer drugs have sometimes had other side effects, however, such as arrhythmia. Therefore, the ideal antihistamine has not yet been developed.

Because of issues such as drug interactions and how the drug may affect the body, antihistamines should be taken cautiously. They also should not be used without the supervision of a physician by women who are trying to become pregnant, by individuals using antibiotics, or by individuals who are undergoing surgery because the effects of the drugs may interfere with these conditions. Other contraindications may exist as well. The advice of a physician or pharmacist is best followed when taking any drug.

Perspective and Prospects

New antihistamines are being developed at a rapid pace. Part of this activity is driven by a desire to make them more effective. For instance, antihistamines have been paired with substances such as decongestants so that they may relieve more symptoms. The development of antihistamines has also been spurred by the desire to decrease the general sedating effects of earlier versions. In the past, however, some additions have come with both advantages and disadvantages. The addition of pseudophedrine to antihistamine medications, for example, increased their ability to address symptoms and decreased some of the drowsiness. Pseudophedrine, however, has stimulating properties that have been identified as problematic and has had to be removed from many types of medications.

Antihistamine development is also driven by a desire to make them more specific. For instance, one person may need an antihistamine to reduce nasal irritation while another may need one to alleviate itchy eyes. As such, it is expected that antihistamines will be developed to have much more specific effects on the body and to have little to no sedating or other side effects while still being effective at reducing allergy and cold symptoms. Nevertheless, some of the familiar antihistamines known today are likely to remain available, as they are generally useful. Also, mildly sedating medicines, such as antihistamines taken at a normally recommended dose, may continue to have a place in medicine, as they serve as a good substitute for individuals needing nonaddictive sleep medication and who are unlikely to abuse these substances.


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