Therapy > Medication > by Category > Antihistamines

Antihistamines

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Biologic effects of histamine:

H1receptor: contraction of endothelial cells and smooth muscle, increase in venule permeability

H2 receptor: secretion of parietal cell acid

 

First generation:

Hydroxyzine (Atarax)

sig:  10mg 1-5 tablets QID
1 QID day #1;  if still itchy take 2 QID day #2;   until 5 QID (i.e.50mg QID)
max daily adult dosage = 400mg

 

Chlorpheniramine = OTC and least sedating first generation anti-histamine

 

First generation:

bind competitively
sedative effect ameliorates in most in a few days of continuous administration
long term therapy appears to be less efficacious over a period of weeks

 

Pharmacokinetics:

efffects observed within 30 minutes, peak in 1-2 hours; persist for 4-6 hours

 

Side effects:

sedation most common
gastrointestinal second most common
contraindicated with MOA inhibitors

 

 

Second generation (low sedating):

Terfenadine (Seldane) off the market; association with torsade de pointes

Astemizole (Hismanal) off the market

Cetirizine (Zyrtec)

metabolite of hydroxyzine
can give as young as age 5
less sedating (take QHS says Marie, and will have no drowsiness next day)
no restriction on taking with other meds
greater activity than hydroxyzine, loratadine and others

Loratadine (Claritin)

no sedative effect compared to placebo
no contraindications even though metabolized by cyt P450

Fexofenadine (Allegra)

 

Second Generation:

bind non-competitively, therefore longer duration (not easily displaced by histamine)
often need to increase dose:
fexofenadine 120mg QID (?)
loratidine or cetiriziine 20mg BID

 

Other:

Doxepin

depression dose = 150mg QHS  (can take several weeks before onset of efficacy)
best H1 blocker?
for itch:  10mg TID (in elderly)  to 30mg TID

 

Aggressive Antihistamine Regimen (from AAD lecture 2011)

fexofenadine 360mg QAM,  loratidine 20mg midday, cetirizine 20mg QHS, and ranitidine 150mg PO BID (all of these are given each day in this regimen)
add singulair 10mg/day (if still not controlled)
"can't hurt someone with non-sedating anti-histamines" per speaker
Soter confirmed at 2011 AAD lecture "up to four times standard dose of all anti-histamines = okay"

 

 

Summary:

antihistamine effect:

Strong: loratadine, fexofenadine, cetirizine HCL, hydroxyzine

Mild: diphenhydramine

anticholinergic effect

Strong: diphenhydramine

Moderate: hydroxyzine

Weak: cetrizine, loratadine, fexofenadine

sedative effect:

Strong: diphenhydramine, hydroxyzine

Occasional: cetirizine HCL

Rare: astemizole, loratadine, fexofenadine