MOA:Antagonise the action of histamine at H1 receptors, reducing histamine-related vasodilation and increased capillary permeability.
They also have anticholinergic activity, some have alpha-blocking activity and some have antiserotonin activity, eg cyproheptadine.
Allergic conditions, eg rhinitis, conjunctivitis, urticaria, contact dermatitis
Nausea and vomiting, including motion sickness
Sedation, eg for cases of burns, measles, chickenpox
Epilepsy—promethazine lowers the seizure threshold.
Pregnancy: Category C, Breastfeeding: Limited data but short-term use appears safe. Sedation of mother is main concern
Closed-angle glaucoma, increased intraocular pressure, pyloroduodenal obstruction, bladder neck obstruction, hyperthyroidism—may be worsened by the anticholinergic effects of antihistamines.
Elderly: Avoid use (less sedating antihistamines preferred for allergic conditions) or use a lower dose; increased risk of adverse effects, eg dizziness, sedation, confusion, hypotension, falls, anticholinergic effects
Children: Avoid use, particularly in children <2 years (less sedating antihistamines preferred for allergic conditions); increased risk of adverse effects, eg sedation, paradoxical stimulation, anticholinergic effects
Common: sedation, dizziness, tinnitus, blurred vision, euphoria, incoordination, anxiety, insomnia, tremor, nausea, vomiting, constipation, diarrhoea, epigastric discomfort, dry mouth, cough
Paradoxical stimulation: CNS stimulation (excitation, hallucinations, ataxia, seizures) may occur rarely, especially in children, rather than sedation.