Second-generation antihistamines
Blocking peripheral H1 receptors while largely staying out of the brain.
Also called non-sedating antihistamines and second-generation H1 antagonists.
Class card
- Drugs
- Cetirizine, Loratadine and Fexofenadine
- Used for
- Hay fever and allergic rhinitis
- Chronic hives (chronic idiopathic urticaria)
- Insect-bite reactions and some food allergies
- Known for
- Headache
- Name stems
- -tadine, -izine
4 cards: the class, and each drug in it.
Mechanism of action
How do second-generation antihistamines work?
Second-generation antihistamines selectively block peripheral H1 receptors, so histamine can no longer make small vessels leak and swell or trigger itching, sneezing and a runny nose. They pass poorly into the brain and have little affinity for other receptors, so at usual doses they cause little drowsiness and almost no anticholinergic effect, and one dose lasts about a day.
The drugs
Second-generation antihistamines with a card
- CetirizineDrowsiness is its commonest side effect (about 10% vs 5% on placebo in trials), a larger excess than reported for loratadine or fexofenadine.
- LoratadineIn psychomotor tests it does not add to the effects of alcohol, unlike first-generation antihistamines.
- FexofenadineBecause it barely enters the brain and is hardly metabolized, it does not sedate at usual doses and has no liver-enzyme interactions.
Uses
What are second-generation antihistamines used for?
- Hay fever and allergic rhinitis
- Chronic hives (chronic idiopathic urticaria)
- Insect-bite reactions and some food allergies
Side effects
Side effects of second-generation antihistamines
- HeadacheKnown for
- Mild drowsiness, most often with cetirizine
- Dry mouth
- Tiredness
- Dizziness
- Nausea
- Fast heartbeat and palpitations (rare)
- Liver function changes or hepatitis (rare)Serious
- Allergic reactions, including anaphylaxisSerious
- Intense itching after stopping long-term cetirizine
What the class has in common. Each drug's card has its own, and Known for marks what an exam expects you to know.
Watch for
Cautions
- Cetirizine is cleared by the kidneys: lower doses in kidney impairment, and none in end-stage kidney disease.
- Loratadine needs a lower starting dose in severe liver disease; CYP3A4 and CYP2D6 inhibitors raise its levels.
- Fruit juice and aluminum or magnesium antacids reduce fexofenadine absorption.
- They are stopped a few days before allergy skin tests, which they would blunt.
Remember them
A way to remember them
The second generation stays out of your head.
Newer antihistamines barely cross the blood-brain barrier and ignore muscarinic receptors, so they ease allergy with little drowsiness or dry mouth; cetirizine is the one most likely to make people sleepy.
Written for biofideX
Name stems
-tadine means tricyclic H1 antihistamine (loratadine type)
Loratadine and desloratadine, H1 antihistamines with a three-ring core, share it. Don't confuse it with -tidine, the H2 blockers; amantadine, an antiviral, fits only by chance.
-izine means piperazine H1 antihistamine (cyclizine type)
Cetirizine, hydroxyzine and cyclizine share it; cetirizine is a metabolite of hydroxyzine. Promethazine, a phenothiazine antihistamine, ends in -azine instead.
- Every drug name stem
Don't mix them up
Classes easily confused with these
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- Cetirizine hydrochloride oral solution: label (DailyMed)
- Cetirizine tablets: SmPC (emc)
- Loratadine tablets: SmPC (emc)
- Fexofenadine tablets: SmPC (emc)
- Cetirizine (NCI Thesaurus)
- Cetirizine (NHS)
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Lungs and allergy
Practice second-generation antihistamines with the other classes in this group: what each drug is, how it works, what it treats and what it's known for.
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