Antihistamines

How they work

Antihistamines are the most widely used drugs to prevent and/or treat allergy symptoms. They block the action of histamine on receptors. Histamine is a chemical released by the body in response to an allergen. It can dilate small blood vessels causing redness, watering and swelling. Receptors that respond to histamine can be found throughout the body. Many are located in the small blood vessels in the skin, nose and eyes. Antihistamines relieve the itching, swelling, hives and redness associated with skin allergies by blocking the action of histamine on these small blood vessels.

When to take antihistamines

Antihistamines are more effective when taken before coming into contact with the allergen. People who can anticipate an allergic reaction because of seasonal allergies or before entering an environment where they know an allergen is present, such as a pet, should take an antihistamine to prevent a reaction.

Antihistamines are not usually effective in asthma brought on by allergies because chemicals other than histamine are also involved in an asthmatic attack. Occasionally, they are used to treat fever, rash, and breathing difficulties that may occur in allergic reactions to drugs.

Products containing antihistamines

Antihistamines may be taken by mouth in liquid or tablet form. They are also available to be applied topically in the form of creams and lotions. Topical preparations often work well in people with eczema. When antihistamines are applied topically it is important to remember that they are absorbed through the skin. Applying too much antihistamine cream over a very large area of the body can result in a toxic dose of the medication, especially if the medication is taken orally as well.

Nasal drops or nasal sprays are sometimes preferred for rhinitis and eye drops are often used when the allergic response is inflammation, redness and swelling of the eyes and/or eyelids.

Side effects of antihistamines

One side effect of antihistamines that can be used as a benefit is drowsiness. People who suffer from eczema often have difficulty sleeping because of the intense itching. Antihistamines that cause drowsiness can help these people sleep better.

Side effects of antihistamines include:

• Drowsiness
• Loss of coordination, clumsiness
• Dry mouth
• Blurred vision
• Difficulty urinating

People who must remain alert for work, study, driving etc. should select an antihistamine that does not cause drowsiness or causes minimal drowsiness, such as cetirizine, desloratidine, fexofenadine or loratadine.

Drug Interactions

Some antihistamines can interact with a large number of medications and some should not be taken if certain other disease states are present. Before choosing an antihistamine, tell your pharmacist what medications you are taking and what disease states you have. They can help you make a selection that is safe for you.

COMPARISON OF ANTIHISTAMINES

	Drowsiness	Non-Drowsy	Treat allergic rhinitis	Treat skin allergies	Duration of action
Azatadine			***	***	over 12 hrs
Brompheniramine	*****		***	***	4-8 hrs
Cetirizine		*	***	***	over 12 hrs
Chlorpheniramine			***	***	4-8 hrs
Cyproheptadine			***	***	4-8 hrs
Desloratidine		*	***	***	over 12 hrs
Diphenhydramine	*****			***	4-8 hrs
Fexofenadine		*	***	***	over 12 hrs
Hydroxyzine	*****			***	4-8 hrs
Loratadine		*	***	***	over 12 hrs
Promethazine	*****		***	***	6-12 hrs

Decongestants help relieve the stuffy nose caused by allergies. They can be bought without a prescription. They may be taken orally as tablets or liquids and are also found in nose sprays, nose drops and eye drops.

Nose sprays and nose drops should not be used for more than 3 days. If one uses these products for longer periods the body becomes dependent on them. When this happens, the body responds with 'rebound congestion'. In other words you become stuffy again when the drug begins to wear off.

Side effects of decongestants

Side effects of decongestants include:

• Increase in blood pressure
• Increase in heart rate, palpitations
• Difficulty sleeping
• Dizziness
• Feeling nervous

Products available

Decongestants available in oral form (tablets, capsules or liquids) include:

• phenylephrine
• pseudephedrine

Drug interactions with decongestants

Decongestants can interact with a large number of medications including:

• anti-anxiety medications
• antidepressants (particularly MAOI inhibiters including isocarboxazid (brand name: Marplan), phenelzine (brand name: Nardil), selegiline (brand names: Eldepryl, Carbex) and tranylcypromine (brand name: Parnate). Pseudoephedrine may cause problems while you are taking an MAOI or within two weeks of taking one.)
• blood pressure medicine
• heart medicine
• diet pills
• asthma medicine
• seizure medicine
• diabetic medicine

Corticosteroids

Corticosteroids can help prevent and treat inflammation caused by allergies. They are available orally in pill and liquid form and topically in creams and lotions, nose sprays and nose drops, eye drops and eye ointments, and inhalers.

Side effects

The side effects of corticosteroids depend on the potency of the medication, the dosage form, the amount absorbed and how long it is used.

Side effects for oral dosage forms may include:

• cataracts
• osteoporosis
• growth suppression

Side effects for creams and lotions may include:

• thinning of the skin
• skin discoloration

Side effects for nose drops and sprays may include:

• unpleasant smell or taste
• nose irritation (especially during the winter)
• crusting and nosebleeds (especially in the winter)

Side effects for eye drops and eye ointments may include:

• blurred vision and eye irritation
• increased risk of eye infections
• increased risk of glaucoma (with prolonged use)
• increased risk of cataracts (with prolonged use)

Leukotriene Modifiers

Leukotriene modifiers block the effects of leukotrienes, inflammatory chemicals released by your immune system during an allergic reaction. These medications help prevent airway tightening, irritation, swelling, and excess mucous production caused by leukotrienes.

Corticosteroids are more effective than leukotriene modifiers. However, these medications may be used when corticosteroids cannot be used or may be added to corticosteroid treatment when asthmatic symptoms continue to be poorly controlled.

Products include:

• Singulair (montelukast sodium)
• Accolate (Zafilukast)
• Zyflo (Zileuton)

Drug interactions

• Warfarin

Side effects

Side effects that may indicate liver disease caused by the medication include:

• poor appetite
• yellow skin
• itching
• stomach pain
• nausea

These side effects should be reported to your health care provider immediately.