Antihistamines
Antihistamine typically refers to drugs that act on and inhibit the activity of the H1 histamine receptor. Antihistamines are commonly used in the treatment of allergic rhinitis, pruritis, skin rashes, insect bites, nausea and urticaria. Antihistamines may also be given in anaphylaxis following initial stabilisation of the patient (note they are very much of secondary importance to giving adrenaline in this respect). They may be broadly classified as sedating (first generation) or non-sedating (second generation) antihistamines.
All older antihistamines cause sedation but alimemazine and promethazine may be more sedating whereas chlorphenamine and cyclizine may be less so. This sedating activity is sometimes used to manage the pruritus associated with some allergies. There is little evidence that any one of the older, 'sedating' antihistamines is superior to another and patients vary widely in their response.
Non-sedating antihistamines such as acrivastine, bilastine, cetirizine, desloratadine (an active metabolite of loratadine), fexofenadine (an active metabolite of terfenadine), levocetirizine (an isomer of cetirizine), loratadine and mizolastine cause less sedation and psychomotor impairment than the older antihistamines because they penetrate the blood brain barrier only to a slight extent.
Sedating antihistamines: key points
The sedating antihistamines are highly lipid-soluble and cross the blood-brain barrier more easily. They are also often of smaller molecular size again aiding crossing of the blood brain barrier.
Sedating antihistamines have significant anti-muscarinic activity and they should therefore be used with caution in prostatic hypertrophy, urinary retention, susceptibility to angle-closure glaucoma, and pyloroduodenal obstruction.
All older antihistamines cause sedation but alimemazine and promethazine may be more sedating whereas chlorphenamine and cyclizine may be less so. This sedating activity is sometimes used to manage the pruritus associated with some allergies. There is little evidence that any one of the older, 'sedating' antihistamines is superior to another and patients vary widely in their response.
Non-sedating antihistamines such as acrivastine, bilastine, cetirizine, desloratadine (an active metabolite of loratadine), fexofenadine (an active metabolite of terfenadine), levocetirizine (an isomer of cetirizine), loratadine and mizolastine cause less sedation and psychomotor impairment than the older antihistamines because they penetrate the blood brain barrier only to a slight extent.
Considerations in the elderly
The use of first-generation antihistamines in elderly patients is potentially inappropriate as safer, less toxic antihistamines are widely available.Sedating and Non-sedating antihistamines
| Sedating | Non-sedating |
|---|---|
| Chlorphenamine Clemastine Alimemazine Cyproheptadine Hydroxyzine Ketotifen Promethiazine Cyclizine | Cetirizine Loratadine Desloratidine Fexofenadine Acrivastine Bilastine Mizplastine |
Sedating antihistamines: key points
The sedating antihistamines are highly lipid-soluble and cross the blood-brain barrier more easily. They are also often of smaller molecular size again aiding crossing of the blood brain barrier.
Sedating antihistamines have significant anti-muscarinic activity and they should therefore be used with caution in prostatic hypertrophy, urinary retention, susceptibility to angle-closure glaucoma, and pyloroduodenal obstruction.