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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 |
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Chlorpheniramine = OTC and least sedating first generation anti-histamine
First generation:
| • | 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:
| • | 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 |
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Loratadine (Claritin)
| • | no sedative effect compared to placebo |
| • | no contraindications even though metabolized by cyt P450 |
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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) |
| • | for itch: 10mg TID (in elderly) to 30mg TID |
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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
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