What Is an Antihistamine?Histamine is a chemical released from allergic cells in the body (such as mast cells and basophils), usually in response to an allergen like cat dander or pollen. When histamine is released by allergic cells in the nose and eyes, the result is sneezing, runny nose, itchy eyes/nose/throat, nasal congestion and post-nasal drip. These are the symptoms of hay fever, also known as allergic rhinitis.
Antihistamines are medications that block a receptor for histamine, thereby stopping the symptoms that histamine causes. Antihistamines are the most commonly used medications to treat allergic rhinitis.
What Are Some Examples of Antihistamines?Older antihistamines, called first-generation antihistamines, include diphenhydramine (Benadryl), chlorpheniramine (Chlor-Trimeton), and hydroxyzine (Atarax/Vistaril). These antihistamines have significant side effects termed “anticholinergic”, which may include dry mouth, sleepiness and urinary retention. Because of the side effects of these medications, they are generally considered to be too sedating for routine daytime use. Therefore, this article will only discuss the newer antihistamines as described below.
Newer antihistamines, called second-generation anthistamines, include cetirizine (Zyrtec), fexofenadine (Allegra, generics), desloratadine (Clarinex), and loratadine (Claritin, Alavert, various generics). These newer antihistamines tend to have fewer anticholinergic side effects, and therefore are termed “low-sedating” or “non-sedating”.
Montelukast (Singulair), is not an antihistamine, but rather an antileukotriene medication. Leukotrienes are chemicals released from a variety of allergic and immune cells, and may cause allergy symptoms, primarily nasal congestion.
What Symptoms of Hay Fever Do Antihistamines Treat?Because antihistamines block the action of histamine, most histamine-causing symptoms are treated by antihistamines. These include sneezing, runny nose, itchy eyes, itchy nose, itchy ears, and itchy throat symptoms. Other symptoms, including post-nasal drip, cough and nasal congestion may also be caused by histamine, and therefore can be treated by antihistamines. However, antihistamines are generally less effective at treating post-nasal drip and nasal congestion symptoms, since other chemicals other than histamine may be involved.
Which Antihistamine Works the Fastest?Various studies have compared the onset of action of antihistamines for treating allergic rhinitis. Two studies by Dr. James Day, sponsored by the makers of Zyrtec, showed that this antihistamine started working in one hour in people allergic to ragweed pollen. Claritin, on the other hand, gave no more benefit than a placebo (sugar pill) until three hours after it was taken.
Which Antihistamine Works the Longest?The same studies by Dr. Day, as described above, showed that once-daily dosing of Zyrtec treated allergic rhinitis for a full 24 hours, whereas Claritin appeared to stop working during the last few hours of a 24-hour study period.
Another study, by Dr. Peter Howarth, compared the effects of once-daily Allegra and Zyrtec on the symptoms of people with grass allergy. The overall effects of the two medications were very similar, and both showed similar improvement of symptoms, even at 24 hours after dosing of these once-a-day antihistamines.
However, an additional study by Dr. Day, comparing the effects of Allegra and Zyrtec on people with ragweed allergy, showed that Zyrtec worked better than Allegra from the 21st to 24th hour after a once-daily dosing of these antihistamines.
Which Antihistamine Works the Best Overall?Yet another study by Dr. James Day compared the effects of Zyrtec and Allegra on the symptoms of allergic rhinitis in people with ragweed allergy. This study looked specifically at hours five to 12 after dosing of the medication. Zyrtec consistently gave better relief than Allegra for these critical “middle hours” after taking an antihistamine.
So, Which Antihistamine Is the Best?The answer to this question is completely based on my experiences and opinions as a board-certified allergist. Studies are very helpful in deciding which medication works best, but it also must be kept in mind which company is paying for the study. It is my opinion that Zyrtec and Allegra are very closely matched, and very good antihistamines. I think these medications work much better than either Claritin or Clarinex.
At the present time, I feel that Zyrtec is the best antihistamine available in the U.S. for the treatment of allergic rhinitis.
Which Antihistamine Causes the Least Amount of Sedation?The topic of sedation is an important one. Sedation refers to the concept that someone feels tired. This is different than impairment, which refers to the concept that someone’s ability to perform various mental and physical tasks is affected.
The only truly non-sedating antihistamine currently available is Allegra. Zyrtec causes sedation more than placebo about five to ten percent more often. Claritin and Clarinex cause minimal sedation.
None of these second-generation antihistamines, when used in recommended doses for allergic rhinitis, have been shown to result in impairment. This is in comparison to the older antihistamines, such as Benadryl, which are well known to result in the impairment of mental and physical tasks.