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Oral Antihistamines: Side Effects for Allergy Drugs

What are oral antihistamines?

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:

(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.

What are examples of oral antihistamines available in the US?

First generation antihistamines include

  • brompheniramine
  • chlorpheniramine (Chlor-Trimeton)
  • diphenhydramine (Benadryl)
  • doxylamine (found in many OTC sleep
    aids including Unisom)
  • carbinoxamine (Karbinal ER)

Second generation antihistamines include


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What are the side effects of oral antihistamines?

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

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

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:

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What drugs interact with oral antihistamines?

Antihistamines may cause drowsiness. Taking antihistamines with other
medicines that also are sedating may cause profound drowsiness. Examples

What formulations of oral antihistamines are available?

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

What about taking oral antihistamines during pregnancy or while

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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