Antihistamines

Histamine is one of the chemicals released when antibodies overreact to allergens. It is the cause of many symptoms of allergic rhinitis. Anti histamines can help relieve:

• Itching, sneezing, and nasal discharge
• Other allergy symptoms unrelated to rhinitis, including hives and some rashes
• Nasal congestion, for some of the newer antihistamines, such as cetirizine (Zyrtec) and desloratadine (Clarinex)

If possible, patients should take antihistamines before an anticipated allergy attack.

Many antihistamines are available. They include short-acting and long-acting forms and are available as tablets, nasal-inhalers, eye drops, and syrups. Antihistamines are generally categorized as first- and second-generation. First-generation antihistamines may cause more side effects than newer second-generation ones.

There are some notes of caution when taking any generation antihistamine:

• Antihistamines may thicken mucus secretions and can worsen bacterial rhinitis or sinusitis.
• Antihistamines can lose their effectiveness over time, and a different one may need to be tried.

First-Generation Antihistamines

First-Generation Antihistamines Ingredients and Brand Names. The older, so-called first generation antihistamines include:

• Diphenhydramine (Benadryl)
• Carbinoxamine (Clistin)
• Clemastine (Tavist)
• Chlorpheniramine (Chlor-Trimeton). Some health professionals recommend this drug if antihistamines are required during pregnancy. It may be as effective as the second generation antihistamines and much less expensive.
• Brompheniramine (Dimetane)

First-generation antihistamines contain compounds called anticholinergics, which tend to produce more side effects than second-generation antihistamines.

Side Effects

• Drowsiness and impaired thinking
• Dry mouth
• Dizziness
• Agitation
• Insomnia or nightmares
• Sore throat
• Rapid heart beat and chest tightness (uncommon and should be reported)
• Men with enlarged prostate glands may experience difficulty urinating

Drowsiness and First-Generation Antihistamines. Drowsiness is the most distressing side effect reported from first-generation antihistamines, and is potentially serious. It may pose a higher than average risk for work-related and automobile accidents than alcohol, narcotics, or prescription sedatives. However, some studies have not found any strong differences in sedation between the first- and second-generation antihistamines.Still, experts caution against the first-generation antihistamines for people most at risk from sedative effects, particularly elderly individuals. To reduce risks, take the antihistamine at home a few hours before bedtime, and do not combine it with alcohol or tranquilizers. Do not drive or operate heavy machinery.

Second-Generation (Nonsedating) Antihistamines

The newer second-generation antihistamines do not contain anticholinergics, so they do not usually cause drowsiness to the extent that the first generation antihistamines do. They are sometimes referred to collectively as nonsedating antihistamines.

A major 2003 analysis reported that although Benadryl, the most common first-generation antihistamine, had a more negative effect on daily activities than the newer antihistamines, the differences were modest. Researchers in the study concluded that no clear distinction exists between the first- and second-generation antihistamines.

Brand Names. The second-generation drugs include:

• Loratadine (Claritin). Claritin is available over-the-counter and is approved for children ages 2 and older. Desloratadine (Clarinex) is similar to Claritin but stronger and longer-lasting. It is available only by prescription.
• Cetirizine (Zyrtec). Zyrtec is approved for both indoor and outdoor allergies. It is the only antihistamine to date approved for infants as young as 6 months. It is available over-the-counter.
• Fexofenadine (Allegra) is also available over-the-counter.
• Acrivastine (Semprex).
• Ebastine, norastemizole, levocetirizine, and mizolastine are other second-generation antihistamines under investigation in the U.S. and Europe. Some may prove to be useful for specific populations.

For nonprescription antihistamines, some studies suggest that cetirizine (Zyrtec) is more effective than Allegra or Claritin in improving symptoms, including those in children. However, cetirizine can cause drowsiness when taken at high doses.

Zyrtec and Claritin are approved for children younger than 5 years, although most antihistamines appear to be safe in children. Zyrtec is the only antihistamine approved for both indoor and outdoor allergies and for infants as young as 6 months. Both are available in syrup form. Studies with Zyrtec have reported fewer symptoms in children allergic to dust mites, and one study reported that infants with allergies who were given Zyrtec were much less likely to develop asthma later on than untreated infants. Claritin, at this time, is generally the preferred drug for young people, however, because it has the least negative effect on concentration and learning. Women who are pregnant or nursing should avoid these medications unless recommended by a doctor.

Side Effects and Precautions

• Common side effects include headache, dry mouth, and dry nose. (These are often only temporary and go away during treatment.)
• Drowsiness occurs in about 10% of adults and between 2% and 4% of children.
• Uncommon side effects include rapid heart beat and chest tightness. Tell your doctor if these effects occur.
• Extended-release forms of Claritin and Zyrtec have other ingredients that can cause other symptoms, including nervousness, restlessness, and insomnia. Some patients taking Claritin-D 24 Hour Extended Release tablets have reported obstruction in the upper gastrointestinal tract, including difficulty swallowing.

Drug and Food Interactions. Two earlier second generation drugs, terfenadine (Seldane) and astemizole (Hismanal), in rare cases, caused dangerous heart rhythm abnormalities, particularly in high doses or in people who had liver disease. They also caused interactions with certain other medications and grapefruit juice. Both Seldane and Hismanal have been taken off the market. Allegra, Zyrtec, and Claritin do not appear to pose any of the dangers associated with Seldane.

Until more is known, however, anyone who takes a second-generation antihistamine should probably avoid or use with caution combinations with grapefruit juice or the drugs that caused problems with Seldane and Hismanal. Such medications include:

• The antibiotics clarithromycin (Biaxin) and troleandomycin
• Certain HIV protease inhibitors
• Antidepressant serotonin-reuptake inhibitors (Prozac, Paxil, and Serzone)

Nasal-Spray Antihistamines

Azelastine (Astelin) and levocabastine (Livostin) are available in nasal spray form. They can reduce nasal congestion as well as allergy symptoms. Both reduce symptoms, although azelastine may be more effective in some patients. Their disadvantages are a bitter taste, drowsiness, and expense. They are not as effective as steroid nasal sprays.

Combination Antihistamines and Decongestants

Many prescription and non-prescription products that combine antihistamines and decongestants are available. Combinations sold over-the-counter include Allerest, Sudafed Severe Cold Formula, Vicks DayQuil, Benadryl Allergy/Sinus, Contac Day/Night Allergy & Sinus. Prescription combinations include Claritin-D, Allegra D, and Zyrtec-D. Symptoms may improve within 60 minutes, with congestion clearing up first..

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Review Date: 3/17/2006
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital.