Updated December 16, 2014.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

What Is an Antihistamine?

Histamine is a chemical that is normally produced in the body and stored in allergic cells, such as mast cells and basophils. In people with allergies, histamine is released from these cells in a response to allergens. Histamine binds to the histamine receptor, which is present on various cells in the body, and results in allergic symptoms such as sneezing, itchy eyes, itchy nose, hives, or even anaphylaxis.

Antihistamines are medications that block the receptor for histamine, thereby stopping the symptoms that histamine causes.
Read the definition of antihistamine.

What Are Antihistamines Used For?

Antihistamines are commonly used for the treatment of various allergic diseases, and can be taken in oral, nasal spray, eye drop, and injected forms.
Antihistamines for Hay Fever (Allergic Rhinitis)

Newer oral antihistamines, such as Xyzal (levocetirizine), Zyrtec (cetirizine), Allegra (fexofenadine), Clarinex (desloratadine) and Clarinex (loratadine), are very effective therapies for the treatment of hay fever. Older antihistamines, such as diphenhydramine (Benadryl), chlorpheniramine (Chlor-Trimeton), and hydroxyzine (Atarax), also work well, but have many side effects. Oral antihistamines work well when taken as-needed, meaning that allergy symptoms improve within an hour or two of taking the medication. Oral antihistamines can also help treat the symptoms of eye allergies.

Antihistamines are also available as nasal sprays for the treatment of allergic rhinitis (nasal allergies) and as eye drops for the treatment of allergic conjunctivitis (eye allergies).

Antihistamines for Hives (Urticaria)

Oral antihistamines are the main treatment for hives. Newer, low-sedating antihistamines, such as Zyrtec or Claritin, are preferred over the older, sedating antihistamines such as Benadryl or Atarax. Topical antihistamine creams, present in many forms of over-the-counter anti-itch creams, should not be used for the treatment of hives or other skin allergies. Placing topical antihistamines on the skin, such as Benadryl cream, can cause contact dermatitis due to the antihistamine.

Other Uses for Antihistamines

Antihistamines are frequently used for the treatment of other allergic conditions, although they may not be as effective as they are for hay fever and hives. These conditions may include:

• The common cold
• Itching of the skin (pruritus)
• Insect stings and bites
• Allergic reactions to medications
• Anaphylaxis

What Are Some Common Oral Antihistamines?

Antihistamines Available Over-the-Counter (OTC)

Probably the most well-known oral antihistamine is Benadryl, which is an older, sedating antihistamine. While Benadryl is a reasonable medication for the treatment of various allergic conditions, side effects limit its routine use, especially during the daytime. Benadryl, and many other older antihistamines, such as Chlor-Trimeton (chlorphenirimine), are available OTC without a prescription.

Learn everything you ever wanted to know about Benadryl.

Two newer, less-sedating antihistamines, Claritin and Zyrtec, are also available OTC without a prescription. These medications cause much less sedation, and are therefore preferred over the older, sedating forms. Allegra will also be available OTC in the Spring of 2011.

Antihistamines Available by Prescription Only

While many older, sedating antihistamines are now available OTC, Atarax is still only available by prescription. Atarax is extremely sedating, and is commonly prescribed for the treatment of skin allergies such as hives and itching. The active metabolite of Atarax, Zyrtec, is available OTC, and causes much less sedation. Xyzal and Clarinex, two less sedating antihistamines, are still available only by prescription. As of the writing of this article in December 2010, Allegra is also only available by prescription, but is due to be available OTC during the Spring of 2011.

What Are the Side Effects of Antihistamines?

Antihistamines and Sedation

Older antihistamines, such as Benadryl and Atarax, have a significant amount of anticholinergic side effects. These side effects may include dry mouth, drowsiness, constipation and urinary retention. Because of the side effects of these medications, they are generally considered to be too sedating for routine daytime use. Since older antihistamines can impair mental and motor functioning, they can impair a person’s ability to operate motor vehicles or heavy machinery. In fact, in many states a person can be charged with driving-under-the-influence (DUI) if they operate a motor vehicle while taking medications such as Benadryl.

Newer, low-sedating antihistamines, such as Claritin and Zyrtec, tend to have fewer anticholinergic side effects. While these newer antihistamines may still cause drowsiness or a dry mouth, they haven’t been shown to impair a person’s ability to operate a motor vehicle. Allegra is the only antihistamine that is truly considered to be non-sedating.

Antihistamines and Weight Gain

In addition to sedation and dry mouth, antihistamines may have the unwanted side effect of increasing appetite and weight gain. This may be due to the similar chemical structure of antihistamines and certain psychiatric medications, such as anti-depressants, which are known to increase appetite and lead to weight gain.

In fact, numerous readers of this site who have used Xyzal reported an increase in appetite and weight gain. The package insert for Xyzal confirms weight gain as a known side effect, but reports this as only occurring in 0.5% of people in studies who were taking this medication. Older antihistamines, such as Periactin (cyproheptadine), have actually been used for the purpose of increasing appetite and weight gain in underweight children and cancer patients undergoing chemotherapy.

Learn about the use of antihistamines in children and during pregnancy.

_{DISCLAIMER: The information contained in this site is for educational purposes only, and should not be used as a substitute for personal care by a licensed physician. Please see your physician for diagnosis and treatment of any concerning symptoms or medical condition.}