Antihistamines can be found over the counter at most drugstores in the U.S. and some types are available by prescription only. They come in a variety of forms including pills, tablets, liquid and powder. Seasonal allergy sufferers often turn to antihistamines to try and find relief from their symptoms.
Uses
Antihistamines are used to prevent an allergic reaction. They are especially useful for seasonal allergies such as hay fever. Antihistamines can be combined with other medicines such as decongestants which treat allergy symptoms. Some antihistamines also have other effects and are used to treat conditions such as insomnia, motion sickness, nausea, vomiting, anxiety and dizziness. Another use for certain types of antihistamines is to relieve tremors in patients with Parkinson's disease.
Biology
Histamines are compounds produced by mast cells---one of the many types of immune cells in the body---in response to allergens, according to author and pediatrician Dr. Alan Greene. Histamine sets off a cascade of activity that leads to the symptoms of allergies such as sneezing, itchy eyes and throat, runny nose and congestion. Antihistamines work by stopping histamine from binding to cells and triggering its biochemical cascade, relieving and preventing the allergic reaction.
Types
According to the American Academy of Family Physicians the two types of antihistamines are first-generation antihistamines and second generation antihistamines. Some examples of first-generation antihistamines are dimenhydrinate, diphenhydramine and doxylamine. These are sold under the brand names Dramamine, Benadryl Allergy, Nytol, Sominex, Vicks NyQuil and Alka-Seltzer Plus Night-Time Cold Medicine. Second-generation antihistamines include loratadine and cetirizine sold as Alavert, Claritin and Zyrtec. The first-generation antihistamines are typically combined with other medications to make cold and allergy formulas, while second-generation antihistamines are typically sold alone specifically to treat seasonal allergies.
Side Effects
First-generation antihistamines are more likely to cause side effects than second-generation antihistamines. Some side effects experienced by users of antihistamines include drowsiness, dry mouth, headaches and abdominal pain. In children, side effects may be more severe and could include nightmares, irritability, restlessness and seizures. Antihistamines may also make children less able to learn and remember new things while they are on the medication, according to Greene. Older adults also tend to have more severe side effects when taking antihistamines.
Contraindications
People who use sedatives, sleeping pills or muscle relaxants should not take antihistamines. Also, individuals with glaucoma, high blood pressure, heart disease, enlarged prostate, asthma, emphysema, kidney disease, liver disease or thyroid problems should consult a doctor before taking antihistamines.
