Antihistamines, Oral

The term, antihistamine generally is used for medications used to treat various symptoms believed to be caused by histamine, for example, as part of the common cold, including:

• Congestion
• Runny nose
• Sneezing
• Itchy throat
• Hives
• Skin rashes
• Itching
• Itchy or watery eyes

(A second type of antihistamine is used primarily for suppressing acid production in the stomach and treating acid-related diseases such as ulcers of the stomach. These antihistamines will not be discussed further.)

Antihistamines also may be used to treat motion sickness, insomnia (difficulty sleeping), and anxiety. Antihistamines work by blocking the effects of a chemical called histamine that is responsible for many allergic symptoms.

Patients who experience significant allergic symptoms regularly may take daily antihistamines to keep their symptoms under control. Antihistamines also can be used on an as needed basis for those who experience occasional symptoms or symptoms triggered by exposure to certain irritants such as animal hair, plants, medications, and food products. Some antihistamines also may be used occasionally to help with sleep.

Many different brands and forms of oral antihistamines are available over the counter (OTC). Oral antihistamines are available as pills, chewable tablets, orally disintegrating tablets, capsules, and liquid. Some antihistamines are found in combination pills that contain other medications. For example, antihistamines are commonly combined with decongestants (for example, Claritin-D, Zyrtec-D, Allegra-D), a class of medicine that is used to dry up the nasal passages and relieve head congestion.

Antihistamines are divided into two categories, first generation or older agents and second generation or newer agents. First generation antihistamines have many drawbacks including side effects of drowsiness and significant anticholinergic side effects that, for example, can cause difficulty urinating or constipation, and are therefore not used very often. The newer antihistamines (second generation) are less likely to cause these side effects.

Antihistamines also differ in how long they work. Long-acting antihistamines provide symptom relief for up to 8-12 hours, while shorter acting agents last for up to 4 hours but begin working faster.

First generation antihistamines are used less often to treat allergies because they cause significant sedation. First generation antihistamines also should be used cautiously in older adults as they are more susceptible to their anticholingeric side effects including

• drowsiness,
• dry mouth,
• decreased tear production,
• urinary retention (trouble urinating),
• blurred vision,
• constipation, and
• agitation.

Due to their significant side effect profile, special precautions should be used in patients with:

• Seizures
• Glaucoma
• Problems passing urine or men with prostate problems
• Thyroid disorders
• High blood pressure
• Diabetes

Second generation antihistamines are less sedating than their first generation counterparts. Cetirizine can be sedating for some patients at normal recommended doses while sedation seems to only be a concern with loratadine at higher than normally recommended doses. Fexofenadine is the least sedating.

Side effects common to all antihistamines include:

• Dizziness
• Dry mouth
• Dry eyes
• Blurred vision
• Problems urinating
• Constipation
• Mental disturbances

• Tablets
• Chewable tablets
• Orally disintegrating tablets
• Capsules
• Solution
• Syrup
• In-combination with decongestants and pain relievers

Non-drug interventions should always be considered first in pregnant women suffering from symptoms of allergic rhinitis. Nondrug therapies include saline or salt-water nasal sprays, exercise, and nasal strips.

If a daily antihistamine is needed during pregnancy, second generation agents are preferred because they are less sedating and have a better side effect profile (have fewer anticholingeric side effects). Loratadine and cetirizine are preferred second generation antihistamines for pregnant women because they have the most safety and efficacy data. Both agents are rated FDA category B, and are generally considered to be safe at recommended doses for the treatment of allergic rhinitis during pregnancy. Fexofenadine is less well-studied and is rated FDA pregnancy category C.

First generation antihistamines are usually used on an as-needed basis or before bedtime to occasionally help with sleep. Chlorpheniramine is the preferred first generation antihistamine for people who are pregnant because it has been studied the most.

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