I’ve developed hives somehow, probably from something in the lamb curry I had for lunch this afternoon.
My mast cells and basophils are in overdrive and this itch is driving me insane! And, the cetirizine is seriously taking its own sweet time to reach peak effect! I am struggling to stop myself from scratching…
Being geeky (and in preparation for an exam), I shall write about chlorphenaramine– a drug used for the same purpose and that I prescribe a lot more commonly.
Use: Allergic rhinitis, Rash (urticaria), Pruritis, Anaphylaxis
Presentation: 4 mg tablets, 0.4mg/ml syrup, 10ml/ml colourless solution for injection
Dose: 4mg oral dose tds/qds, 10mg iv/im/sc dose
Mechanism of axn: Antihistamine (competitive antagonism of H1 receptors), Anticholinergic
– CVS: Inhibits histamine induced vasodilatation and capillary permeability.
– RS: Reduction of bronchial secretions, doesnt fully reverse anaphylactoid bronchospasm as leukotrienes are also involved in allergic bronchoconstriction.
– GI: N+V, other GI disturbances
– CNS: Sedating, some LA properties
– Anticholinergic properties
– Absorption: Slow absorption, 25-50% oral bioavailability (due to extensive first pass metabolism)
– Distribution: 70% plasma protein bound, Vd approx 7.5L/ kg
– Metabolism: Liver- oxidative deamination, demethylation
– Excretion: Urine, Cl 4.4-7.9ml/kg/min, elimination T1/2= 2-43hr. Not removed by dialysis.