Antihistamines Flashcards
Give 3 examples of non-sedating antihistamines
- Cetirizine
- Fexofenadine
- Loratidine
Give 3 examples of sedating antihistamines
- Chlorphenamine
- Promethazine
- Cyclizine
What is the mechanism of action of antihistamines?
They compete with histamines for binding to the histamine H1 receptors, thus blocking the effects of histamine
What do histamines do?
They cause vasodilation and increased vascular permeability
What happens in anaphylaxis, regarding histamine?
Histamines are released and vasodilation is widespread and results in significant hypotension
What are the routes of delivery of antihistamines?
- PO
- IV
When might antihistamines be given IV?
In anaphylaxis
What are the indications for antihistamines?
- Emergency treatment of anaphylaxis
- Pruritis and urticaria
- Seasonal allergic rhinitis
- Anti-emetic
- Insomnia
What antihistamine is used in the emergency treatment of anaphylaxis?
Chlorphenamine
What antihistamine is most effect in treating urticaria and pruritus?
Chlorphenamine
What antihistamine is most commonly used as an anti-emetic?
Cyclizine
When should antihistamines be used with caution?
- Prostatic enlargement
- Urinary retention
- Pyloric stenosis
- Pyloroduodenal obstruction
- Epilepsy
- Patients at risk of angle-closure glaucoma
Can antihistamines be used during pregnancy and breastfeeding?
Should check advice from manufacturer before taking specific antihistamine during pregnancy and breastfeeding
What monitoring should be done with antihistamines?
- Monitor patients for symptomatic improvement
- Follow local protocols for post-anaphylaxis monitoring
What might antihistamines interact with?
- TCAs and MAOIs
- Muscarinic antagonists
What might happen if you give TCAs/MAOIs with antihistamines?
Increased sedation and anti-muscarinic effects
What might happen if you give muscarinic antagonists with antihistamines?
Antimuscarinic effects become more pronounced
What are the common side effects of antihistamines?
- Headache
- Drowsiness
- Antimuscarinic effects
What are the less common side effects of antihistamines?
- Extrapyramidal side effects
- Hypersensitivity reactions
- Psychomotor impairment
What counselling is required with antihistamines?
- Carry with you
- Drowsiness and impairment of function
What counselling is required with antihistamines regarding carrying them with you?
Advise patients to carry antihistamines with them if they suffer from allergic reactions, particularly in cases where specific causative agents are unknown
Other than antihistamines, what should a patient with a severe allergy or history of anaphylaxis carry with them?
Adrenaline auto-injection pen
What counselling is required with antihistamines regarding drowsiness and impairment of function?
Warn patients about drowsiness, as this may impair the patient ability to drive or operate machinery. This is not advised when taking sedating anti-histamines, and the patient should follow specific instructions from the manufacturer of the antihistamines they are taking regarding driving safety and the use of machinery
