Antihistamines for Dermatology Board Preparation

Classification and Mechanism

Antihistamines are H1 receptor antagonists that prevent histamine from binding to H1 receptors in the skin, thereby blocking the primary symptom of itch and reducing allergic inflammation. 1

First-Generation Antihistamines (FGAs)

Highly lipophilic agents that readily cross the blood-brain barrier, causing significant sedation, cognitive impairment, and anticholinergic effects (dry mouth, tachycardia, urinary retention, constipation). 2, 1
Examples include diphenhydramine, hydroxyzine, chlorphenamine, and alimemazine 2
Performance impairment can occur without subjective awareness of drowsiness—a critical safety concern. 2
Sedation potential persists longer than plasma drug levels would predict. 2
Not recommended for routine use in allergic diseases due to adverse effect profile. 1

Second-Generation Antihistamines (SGAs)

Minimally sedating, longer-acting, and better tolerated than FGAs, making them first-line therapy for allergic diseases. 1, 3
Examples include cetirizine, loratadine, desloratadine, fexofenadine, and levocetirizine 2, 4
Non-brain-penetrating antihistamines (nbpAHs) represent a reclassification within SGAs based on H1 receptor occupancy studies using PET imaging—these are rapidly acting with minimal adverse reactions. 1

Clinical Applications in Dermatology

Chronic Spontaneous Urticaria (CSU)

Start with standard-dose second-generation antihistamines as first-line therapy. 5

Treatment Algorithm:

If inadequate control after 2-4 weeks (or earlier if symptoms are intolerable), increase the second-generation antihistamine dose up to 4-fold the standard dose. 5
If still inadequate after dose escalation, add omalizumab 300 mg subcutaneously every 4 weeks as second-line therapy. 5
If omalizumab fails within 6 months (or earlier if intolerable), add ciclosporin up to 5 mg/kg body weight as third-line therapy. 5

Step-Down Protocol:

In patients with complete disease control (UCT score >16), consider step-down to reduce treatment burden and assess for spontaneous remission. 5
Do not step down from higher-than-standard dosing until at least 3 consecutive months of complete control. 5
Reduce daily dose by no more than 1 tablet per month. 5
If breakthrough symptoms occur during step-down, return to the last dose that provided complete control. 5

Symptomatic Dermographism

Second-generation H1-antihistamines should be first-line treatment for symptomatic dermographism, as they are effective and well-tolerated in all studies. 6

First-generation antihistamines showed variable efficacy and significant side effects in dermographism. 6
Adding an H2-antihistamine to H1-antihistamine therapy may increase efficacy, though H2-antihistamines alone are not effective. 6
Omalizumab improves symptomatic dermographism in refractory cases. 6
No evidence exists for updosing second-generation antihistamines in dermographism, but it may be considered by extrapolating evidence from chronic spontaneous urticaria. 6

Atopic Dermatitis (AD)

Antihistamines are NOT generally recommended for atopic dermatitis management, as there is no high-level evidence that nonsedating antihistamines reduce itch in AD patients. 7

The American Academy of Dermatology does not recommend general use of antihistamines in AD management. 7
Short-term sedating antihistamine use may have value specifically for insomnia secondary to itch—this is the only recognized indication. 7
A Cochrane systematic review found no consistent evidence that H1 antihistamines as add-on therapy to topical treatment improve patient-assessed symptoms, physician-assessed clinical signs, or quality of life in eczema. 8
Fexofenadine 120 mg/day showed only a small reduction in pruritus (mean difference -0.25 on 0-8 scale) that may not be clinically meaningful. 8
Cetirizine and loratadine showed no benefit over placebo for AD symptoms. 8

Allergic Rhinitis

For mild intermittent or mild persistent allergic rhinitis, use a second-generation oral H1 antihistamine (cetirizine, fexofenadine, desloratadine, loratadine) or intranasal antihistamine (azelastine, olopatadine) as first-line therapy. 4

For persistent moderate-to-severe allergic rhinitis, initiate treatment with an intranasal corticosteroid alone or in combination with an intranasal antihistamine. 4
Intranasal corticosteroids and non-sedating antihistamines are first-line due to high efficacy and safety. 9
Intranasal antihistamine plus intranasal corticosteroid combinations are preferred over either agent alone. 10

Special Populations

Older Adults

Second-generation antihistamines (fexofenadine, loratadine, desloratadine) are strongly preferred in older adults due to less sedation and fewer anticholinergic effects. 2

First-generation antihistamines pose significant risks in older adults including falls, confusion, urinary retention, and cognitive impairment due to anticholinergic burden. 2, 11
Anticholinergic effects are particularly problematic in patients with narrow-angle glaucoma. 2

Pregnancy

When antihistamine therapy is necessary during pregnancy, cetirizine or loratadine should be used as they are FDA Pregnancy Category B drugs with the best safety profile. 12

Key Principles:

Ideally avoid all antihistamines during pregnancy, especially in the first trimester, though none has been proven teratogenic in humans. 12, 13
Hydroxyzine is specifically contraindicated during early pregnancy and should never be used. 12, 13
Chlorphenamine is often selected by UK clinicians when antihistamine therapy is necessary due to its long safety record, despite being a first-generation antihistamine. 12, 13
Large meta-analyses of approximately 200,000 first-trimester exposures to first-generation antihistamines found no association with increased congenital anomalies. 13

Pregnancy Treatment Algorithm for Urticaria:

Start with standard doses of cetirizine or loratadine 12
If inadequate control after 2-4 weeks, increase dose up to 4-fold, weighing benefits against risks 12
For refractory cases, consider omalizumab 300 mg every 4 weeks (limited pregnancy data but EXPECT registry shows no increased adverse outcomes with first-trimester exposure) 12
For severe acute exacerbations only, use short courses (3 days) of oral prednisolone, which is 90% inactivated by the placenta 12

Critical Pregnancy Warnings:

Avoid combining antihistamines with oral decongestants during first trimester due to associations with gastroschisis and small intestinal atresia. 13
Systemic corticosteroid use in first trimester is associated with 3-fold increased risk of isolated cleft lip ± cleft palate. 12
Neonatal withdrawal syndrome has been associated with hydroxyzine use later in pregnancy, with documented tremors, irritability, and hyperactivity lasting up to 5 weeks. 13

Renal Impairment

Dose adjustments are required for several antihistamines in renal impairment. 2

Acrivastine should be avoided in moderate renal impairment (CrCl 10-20 mL/min). 2
Cetirizine, levocetirizine, and hydroxyzine doses should be halved in moderate renal impairment. 2
Cetirizine, levocetirizine, and alimemazine should be avoided in severe renal impairment (CrCl <10 mL/min). 2
Loratadine and desloratadine should be used with caution in severe renal impairment. 2

Hepatic Impairment

Several antihistamines require caution or are contraindicated in hepatic impairment. 2

Mizolastine is contraindicated in significant hepatic impairment. 2
Alimemazine should be avoided due to hepatotoxicity risk. 2
Chlorphenamine and hydroxyzine should be avoided in severe liver disease due to inappropriate sedating effects. 2

Pediatric Considerations

No currently licensed antihistamines are contraindicated in children 12 years and older, but dosing and age restrictions vary for younger children. 2

A quality improvement initiative in a children's hospital successfully reduced first-generation antihistamine use from 74% to 28% in the pediatric emergency department by promoting cetirizine use. 3
Second-generation antihistamines are equally effective, longer-acting, and better tolerated than first-generation agents in children. 3

Practical Prescribing Considerations

When Sedation is Beneficial

For nighttime use when sedation is beneficial for sleep, consider first-generation antihistamines. 2

Doxepin has useful antihistaminic properties but has sedating and anticholinergic side effects, making it appropriate for nighttime use when sedation aids sleep. 2

Allergy Testing

Antihistamines with anti-histamine activity must be discontinued several days before skin testing. 2

Pre-treatment with antihistamines will not reduce the severity of anaphylaxis. 2
There is no evidence supporting the use of H2-blocking drugs in immediate management of anaphylaxis. 2

Monitoring and Adverse Effects

Monitor for adverse effects including sedation, falls, confusion, dry mouth, constipation, and urinary retention, particularly in older adults. 2

First-generation antihistamines have significant anticholinergic effects that are particularly problematic in older adults and those with narrow-angle glaucoma. 2
Performance impairment can occur without subjective awareness of drowsiness. 2

Common Pitfalls and Caveats

Inappropriate Use in Atopic Dermatitis

Antihistamines are widely prescribed for AD (16-44% of visits across specialties) despite lack of evidence for efficacy. 7
Dermatologists and pediatricians primarily use sedating antihistamines (58-70%) for AD, while family practitioners use nonsedating antihistamines (55-100%), but neither approach has strong evidence. 7

Polypharmacy in Older Adults

Consider anticholinergic burden and polypharmacy when prescribing antihistamines to older adults. 11
Review all medications for cumulative anticholinergic effects. 11

Pregnancy Prescribing Errors

Never prescribe hydroxyzine during pregnancy—it is the only antihistamine specifically contraindicated. 12, 13
Avoid diphenhydramine as first-line treatment despite frequent use, as it has been associated with cleft palate development. 12

Can you provide study notes on antihistamines for dermatology board preparation based on Fitzpatrick Dermatology, 9th edition, chapter 189?

Help us tailor your experience