shevyatiwari's version from 2015-10-23 10:00


Question Answer
H1 and H2 are inverse agonistsT. Downregulate constitutive activated state of the corresponding receptor


First generation
Question Answer
H1 should be used regularlyT
Bind to H1 or H2 receptorsF bind to H1, but no significant binding to H2 or H3
There are 4 classes of H1 antihistaminesF, 5.
Are lipophilicT
Cross the BBBT
Peak plasma levels in 1-2 hoursT
H2 antihistamines are second generationF, first
Ketoconazole has no effect on first generation antihistaminesF, prolongs half life
Half life in skin is much greater than plasma half lifeT
Cyproheptadine is the preferred antihistamine for cold an other physical urticariasT. Also has anti-serotonin properties
Cyproheptadine is safe in kidsF, may retard growth
Tend to undergo more hepatic metabolism than second generationT
Most are excreted in the urineT
CYP3A4 inhibitors cause increased risk of torsadesT


Second generation
Question Answer
Narrow therapeutic indexF. Second Gen antihistamine have a high therapeutic inde- ration of min toxic dose and min therapeutic dose - hence ok to give QID off label safely
High doses have no proved efficacyT
Are lipophilicF
Can cross the BBBF
Moderate anticholinergic activityF, little
Fexofenadine is a pro drugF, terfenadine is - it is cardiotoxic
Fexofenadine undergoes significant hepatic metabolismF
Fexofenadine is excreted in the bileT, excreted largely unchanged in the bile 80%. FEXofendine = Faecal
Dose adjustment of fexofenadine should be considered in the elderly or those with hepatic impairmentF
Loratadine is a pro drugT
Loratadine produces an active metaboliteT, desloratadine
Desloaratadine is 10 times more potent than loratadineF, 5 times
Desloaratdine does not interact with any medications just like loratadineT
Tolerance is possible with loratadineF
Chronic renal or hepatic impairment requires dose adjustment of loratadineT officially but renal impairment, hepatic impairment and age have no major influence on the drugs pharmacokinetics
CYP3A4 inhibitors have no effect on loratadineT
Cetirizine is a pro drugF, is a metabolite. Certirizine is the carboxylic acid metabolite of first gen H1 antihistamine Hydroxyzine. undergoes minimal metabolic transformation to inactive metabolite and excreted unchanged in the urine
Cetirizine is excreted largely unchanged in the urineT. CetirizINE - URINE - UNCHANGED!
Cetirizine has problems with toleranceF
Renal or liver disease warrants dose reduction of cetirizineT
Cetirizine is the least sedating H2 antihistamineF, the most sedating
Cetirizine has no significant drug interactionsT
Cetirizine has an inactive metabolite levocetirizine which can also be given to patientsF, active metabolite
Safer than first generation in pregnancyF, other way around


Question Answer
Doxepin is a TCA with only H1 activityF, also H2. Tricyclic antidepressant with potent H1 AND H2 activity
Doxepin is available only in topical formulationF
Drowsiness is a S/E of topical DoxepinT significant drowsisness can result form percutaneous absorption.. only use for 8 days
Allergic contact dermatitis is a S/E of topical DoxepinT. Dermatitis medicamentosa
Doxepin is safe with other antidepressants as long as it is used topicallyF - DO NOT GIVE WITH OTHER ANTIDEPRESSANTS
Severe heart disease is not a C/I to DoxepinF


Question Answer
H1 receptors only are found in the skinF, also H2
Histamine is synthesised by histidine from histidine decarboxylaseT
Itching and flare are mediated by H2F, H1
T lymphocyte activity is mediated by H2T
Elevated histamine is seen in those with chronic urticariaT
Vasodilatation and increased permeability are mediated by H2F, H1 AND H2

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