chemical released by the body
involved in CNS transmission, dilation of capillaries, contraction of smooth muscles, stimulation of gastric secretion, and increased heart rate
two cellular receptors for histamine
H1 receptors - mediate smooth muscles in the capillaries
H2 receptors - mediate the acceleration of the heart rate and gastric acid secretion
hay fever, mold, dust allergies
drugs that directly compete with histamine for receptor sites
antagonist/blockers; anticholinergic
H1 receptors are throughout the body
many are available OTC, although some are available by prescription only
they are extremely useful because approximately 10-20% of the population is sensitive to various environmental allergies
Benadryl, Zyrtec, Dimetane, Tavist, Dramamine, Atarax/Vistaril, Antivert, Phenergan, Unisom
work by blocking the action of histamine throughout the body
do not push histamine off but rather competes with histamine for receptor sites
provides symptomatic relief but is not curative
more effective if given early in reaction
prevents dilation of blood vessels and therefore decreases permeability of blood vessels
reduces edema, hives, and urticaria
prevents salivary, gastric, lacrimal, and bronchial secretions
has a drying effect and reduces runny nose, teary and itchy eyes
has anticholinergic effects
blocks acetylcholine receptors in the PNS and prevents transmission of impulses
dry mouth, changes in vision, difficulty urinating, constipation, GI disturbances
drying effect (anticholinergic)
dysrhythmias, arrest, hypotension, palpitations, syncope, dizziness
Allegra levels are increased if given with erythromycin
Claritin levels are increased by cimetidine and erythromycin
Benadryl and Zyrtec interact with MAOIs and also alcohol and cause increased CNS depressant effects
warn patient that drug may cause drowsiness
caution against activities that require mental alertness
alcohol and other CNS depressant drugs may potentiate CNS depression and should be avoided
administer with meals to reduce GI disturbances
Non-sedating Antihistamines
work peripherally to block the actions of histamine and therefore do not have the CNS effects that many older antihistamines have
peripherally acting antihistamines
longer duration of action (can be taken once a day)
encourage patient to contact MD if excessive sedation, confusion, or hypotension occurs
avoid driving or operating heavy machinery
do not consume alcohol or take other CNS depressants
educate regarding medication, SE, drugs with which they interact
patient should always tell MD that they are taking non-sedating antihistamines because of the adverse reactions with erythromycin and ketonazole
always check the package inserts that come with OTC cold preparations to find out the drugs they interact with
