Antihistamines
Histamine is a chemical mediator released by mast cells and basophils. It plays a central role in urticaria (hives), prurigo, atopic dermatitis and other allergic skin reactions. H1 antihistamines are among the most frequently prescribed drugs in dermatology for controlling itch and swelling. Current pharmacology recognizes that they are not simply competitive blockers: they act as inverse agonists, locking the H1 receptor in its inactive state.
At Haute-Derma, we prescribe H1 antihistamines in line with international guidelines (EAACI/GA²LEN/EDF/WAO). We favor non-sedating second-generation drugs, use up-dosing to as much as four times the standard dose in refractory chronic urticaria, and tailor itch control to each patient.
How H1 Antihistamines Work
Inverse Agonism
The H1 receptor is a G protein-coupled receptor (GPCR) that constantly shifts between an active and an inactive state. When histamine binds, the balance tips toward the active state. H1 antihistamines bind preferentially to the inactive form and hold it there, lowering the receptor's baseline activity even before any free histamine arrives.
Clinical Effects on the Skin
- Less vascular leakage: By occupying H1 receptors on endothelial cells, they prevent vessel dilation and plasma leakage, so wheals and angioedema settle quickly.
- Calmer itch nerves: Blocking H1 receptors on afferent sensory C fibers shuts down the itch signal before it reaches the cerebral cortex.
- Additional anti-inflammatory effects (second generation): At therapeutic concentrations, some second-generation antihistamines (such as rupatadine, ketotifen and bilastine) also help stabilize mast cells, reducing the release of secondary mediators such as leukotrienes, PAF and pro-inflammatory cytokines (IL-4, IL-13, TNF-alpha).
Generations of Antihistamines in Clinical Use
- First generation (sedating, classical): hydroxyzine, diphenhydramine, chlorpheniramine. These are highly lipophilic, cross the blood–brain barrier easily and bind central H1 receptors, causing marked drowsiness and poor concentration. They also have anticholinergic side effects (dry mouth, urinary retention, blurred vision, constipation).
- Second generation (non-sedating): cetirizine, loratadine, fexofenadine. They bind selectively to peripheral H1 receptors and cross the blood–brain barrier poorly. Their long half-lives allow once-daily dosing, and at standard doses they cause little or no drowsiness.
- Newer second generation (metabolites and enantiomers): desloratadine, levocetirizine, bilastine, rupatadine. These are active metabolites or pure isomers, with a fast onset and very high receptor affinity. Bilastine and fexofenadine in particular undergo little or no hepatic CYP450 metabolism, so they place minimal load on the liver and have very few drug interactions.
Comparing Common Antihistamines
| Drug (Generic) | Drowsiness | Onset and Duration |
|---|---|---|
| Cetirizine (10 mg) | Mild (dose-dependent) | Onset 20–60 minutes, lasts 24 hours |
| Levocetirizine (5 mg) | Very low | Twice the receptor affinity of cetirizine, lasts 24 hours |
| Fexofenadine | Non-sedating | Safe for drivers and machine operators |
| Bilastine (20 mg) | Non-sedating | No central nervous system effects, liver-friendly |
| Desloratadine (5 mg) | Non-sedating | Strong additional anti-inflammatory effect, lasts over 24 hours |
| Hydroxyzine (25 mg) | Very high | Deep sedation, anticholinergic, anxiolytic |
Up-Dosing in Chronic Urticaria and Managing Side Effects
Up-Dosing (Two to Four Times the Standard Dose)
For chronic spontaneous urticaria (CSU) or physical urticaria that is not controlled by the standard one tablet a day after 2–4 weeks, the international guideline (EAACI/GA²LEN/EDF/WAO) recommends:
- Increasing the second-generation H1 antihistamine to two, three and up to four times the daily dose (for example, bilastine 20 mg, 2–4 tablets a day, or levocetirizine 5 mg, 2–4 tablets a day) before considering the biologic omalizumab or the immunosuppressant cyclosporine.
- Clinical studies show that up-dosing second-generation drugs markedly improves control of wheals and itch without adding dangerous side effects.
Side Effects and Food Interactions
- Risks of first-generation drugs: They disrupt REM sleep architecture, dull thinking the next day and increase the risk of falls in older adults.
- Specific food interactions: Bilastine and fexofenadine are absorbed much less when taken with food or fruit juice (grapefruit, orange, apple). Bilastine must be taken on an empty stomach (1 hour before or 2 hours after a meal).
Our Treatment Pathway at Haute-Derma
Assessing the Cause and Severity of Itch
- Severity is measured with the Urticaria Activity Score (UAS7) and a visual analog scale (VAS) for itch.
- We distinguish histamine-driven itch (urticaria, contact allergy) from non-histaminergic itch (liver cirrhosis, kidney failure, chronic atopic dermatitis) so the right add-on treatments can be combined.
Stepwise Treatment
- Step 1: A second-generation H1 antihistamine at the standard dose of one tablet a day.
- Step 2: Up-dosing to two to four times daily if symptoms remain uncontrolled after 2–4 weeks.
- Step 3: Consider adding low-dose hydroxyzine at night if itch is severely disturbing sleep, or switch to omalizumab if the urticaria is fully refractory.
Monitoring and Optimizing Medication
- Clear advice on when to take each drug for the best possible absorption.
- UAS7 is reassessed every 4 weeks, and the dose is stepped down gradually once the patient has been completely controlled (UAS7 = 0) for 3 consecutive months.
