Antihistamines and itch — Viva

Q1: Inverse agonism (2 min)

Define an H1 antihistamine and explain why it is described as an inverse agonist rather than a simple competitive antagonist.

Model answer. H1-receptors are GPCRs whose inactive and active conformations coexist in equilibrium; constitutive activity is receptor activation in the absence of histamine. Histamine stabilises the activated conformation. Inverse agonists combine with and stabilise the inactive conformation, shifting the equilibrium towards the inactive state, and may down-regulate constitutive activity even without histamine. Neutral antagonists combine equally with both conformations. ^[1]

Q2: First-generation versus second-generation (3 min)

Why are first-generation H1 antihistamines avoided for chronic urticaria, and which newer agents have the least sleepiness at usual doses?

Model answer. CSACI: first-generation agents (diphenhydramine, hydroxyzine) cause sedation, decreased cognitive function, poor sleep quality, dry mouth, dizziness, and orthostatic hypotension, and have been found to result in death from accidents, overdoses, and sudden cardiac death. They should be used only as a last resort. Newer-generation agents are first-line for allergic rhinitis and urticaria. Leelakanok: the least-sedating antihistamine in adults was bilastine 20 mg (sleepiness 1.6%); bepotastine 20 mg, bilastine 20 mg, desloratadine 5 mg, and fexofenadine 120 to 180 mg had sleepiness under 10% and not significantly different from placebo. ^[2][8]

Q3: Cardiotoxicity (3 min)

What is the molecular mechanism by which some H1-antihistamines may cause cardiac arrhythmias?

Model answer. Leurs: blockade of HERG1 K+ channels is the mechanism by which some H1-antihistamines may cause cardiac arrhythmias; that finding allowed preclinical tests to predict such activity. Named withdrawn-drug lists and CYP3A4–macrolide recipes are not in the fetched abstracts. Wang: bilastine has no cardiotoxicity. ^[1][10]

Q4: Chronic spontaneous urticaria ladder (3 min)

A 35-year-old woman has CSU not controlled by a standard-dose second-generation H1 antihistamine. What is the next step?

Model answer. Do not switch to a first-generation agent. Current guidelines recommend step-up second-generation H1-antihistamines to four-fold the approved dose, then omalizumab, then ciclosporin. Saini (n=90, single subcutaneous dose): 300 mg and 600 mg improved UAS7 versus placebo; 75 mg did not; onset after 1 to 2 weeks. Kolkhir JAMA: at least 30 percent have an insufficient omalizumab response; ciclosporin (off-label) improves approximately 54 to 73 percent. ^[3][4]

Q5: Anaphylaxis (3 min)

What is the place of antihistamines in anaphylaxis?

Model answer. EAACI: prompt intramuscular adrenaline is first-line management, with adrenaline autoinjectors available to patients in the community. AAP: prompt intramuscular epinephrine in the mid-outer thigh reduces hospitalizations, morbidity, and mortality. Antihistamines are not the first-line resuscitation drug in these abstracts. Unsourced millilitre-per-kilogram adrenaline recipes are omitted. ^[5]

Q6: Non-histaminergic itch (2 min)

Give two pruritic conditions in which antihistamines have limited efficacy and what you would use instead.

Model answer. Atopic dermatitis: oral antihistamines are not recommended because they do not reduce pruritus (Frazier); first-line flares are topical corticosteroids, with topical calcineurin inhibitors usable alongside. Chronic pruritus generally (Butler): inflammatory (~60 percent) — hydrocortisone 2.5 percent, triamcinolone 0.1 percent, or tacrolimus; neuropathic (~25 percent) — gabapentin, sertraline or doxepin, naltrexone or butorphanol. About 10 percent fail topicals. ^[6][7]

Q7: Older adults and renal impairment (2 min)

How do older age and renal impairment alter antihistamine choice?

Model answer. Pelen: antidepressants, antihistamines, and antimuscarinics were the most common anticholinergic classes in the two studies using that classification; greater anticholinergic burden is associated with physical and cognitive impairment. CSACI: first-generation last resort. Wang: bilastine does not require dose adjustment in renal impairment and hepatic impairment is not expected to increase exposure above its safety margin. ^[9][2][10]

References10ShowHide

1. [1]Leurs R, Church MK, Taglialatela M. H1-antihistamines: inverse agonism, anti-inflammatory actions and cardiac effects Clin Exp Allergy, 2002.PMID 11972592
2. [2]Fein MN, Fischer DA, O'Keefe AW, et al. CSACI position statement: Newer generation H1-antihistamines are safer than first-generation H1-antihistamines and should be the first-line antihistamines for the treatment of allergic rhinitis and urticaria Allergy Asthma Clin Immunol, 2019.PMID 31582993
3. [3]Zuberbier T, Ensina LF, Giménez-Arnau A, et al. Chronic urticaria: unmet needs, emerging drugs, and new perspectives on personalised treatment Lancet, 2024.PMID 39004090
4. [4]Saini S, Rosen KE, Hsieh HJ, et al. A randomized, placebo-controlled, dose-ranging study of single-dose omalizumab in patients with H1-antihistamine-refractory chronic idiopathic urticaria J Allergy Clin Immunol, 2011.PMID 21762974
5. [5]Muraro A, Worm M, Alviani C, et al. EAACI guidelines: Anaphylaxis (2021 update) Allergy, 2022.PMID 34343358
6. [6]Frazier W, Bhardwaj N. Atopic Dermatitis: Diagnosis and Treatment Am Fam Physician, 2020.PMID 32412211
7. [7]Butler DC, Berger T, Elmariah S, et al. Chronic Pruritus: A Review JAMA, 2024.PMID 38809527
8. [8]Leelakanok N, Pongpun A, Sapapsap B, et al. The incidence of sleepiness in newer-generation antihistamine users: a systematic review and network meta-analysis Eur J Clin Pharmacol, 2026.PMID 42481869
9. [9]Pelen K, Hagenimana WB, Baroud ML, et al. Prevalence and factors associated with anticholinergic medication use in community-dwelling older adults: a systematic review Int J Clin Pharm, 2025.PMID 41066035
10. [10]Wang XY, Lim-Jurado M, Prepageran N, et al. Treatment of allergic rhinitis and urticaria: a review of the newest antihistamine drug bilastine Ther Clin Risk Manag, 2016.PMID 27110120