Antihistamines

Question 1

Give 3 examples of non-sedating antihistamines

Answer 1

• Cetirizine
• Fexofenadine
• Loratidine

Question 2

Give 3 examples of sedating antihistamines

Answer 2

• Chlorphenamine
• Promethazine
• Cyclizine

Question 3

What is the mechanism of action of antihistamines?

Answer 3

They compete with histamines for binding to the histamine H1 receptors, thus blocking the effects of histamine

Question 4

What do histamines do?

Answer 4

They cause vasodilation and increased vascular permeability

Question 5

What happens in anaphylaxis, regarding histamine?

Answer 5

Histamines are released and vasodilation is widespread and results in significant hypotension

Question 6

What are the routes of delivery of antihistamines?

Answer 6

• PO

- IV

Question 7

When might antihistamines be given IV?

Answer 7

In anaphylaxis

Question 8

What are the indications for antihistamines?

Answer 8

• Emergency treatment of anaphylaxis
• Pruritis and urticaria
• Seasonal allergic rhinitis
• Anti-emetic
• Insomnia

Question 9

What antihistamine is used in the emergency treatment of anaphylaxis?

Answer 9

Chlorphenamine

Question 10

What antihistamine is most effect in treating urticaria and pruritus?

Answer 10

Chlorphenamine

Question 11

What antihistamine is most commonly used as an anti-emetic?

Answer 11

Cyclizine

Question 12

When should antihistamines be used with caution?

Answer 12

• Prostatic enlargement
• Urinary retention
• Pyloric stenosis
• Pyloroduodenal obstruction
• Epilepsy
• Patients at risk of angle-closure glaucoma

Question 13

Can antihistamines be used during pregnancy and breastfeeding?

Answer 13

Should check advice from manufacturer before taking specific antihistamine during pregnancy and breastfeeding

Question 14

What monitoring should be done with antihistamines?

Answer 14

• Monitor patients for symptomatic improvement

- Follow local protocols for post-anaphylaxis monitoring

Question 15

What might antihistamines interact with?

Answer 15

• TCAs and MAOIs

- Muscarinic antagonists

Question 16

What might happen if you give TCAs/MAOIs with antihistamines?

Answer 16

Increased sedation and anti-muscarinic effects

Question 17

What might happen if you give muscarinic antagonists with antihistamines?

Answer 17

Antimuscarinic effects become more pronounced

Question 18

What are the common side effects of antihistamines?

Answer 18

• Headache
• Drowsiness
• Antimuscarinic effects

Question 19

What are the less common side effects of antihistamines?

Answer 19

• Extrapyramidal side effects
• Hypersensitivity reactions
• Psychomotor impairment

Question 20

What counselling is required with antihistamines?

Answer 20

• Carry with you

- Drowsiness and impairment of function

Question 21

What counselling is required with antihistamines regarding carrying them with you?

Answer 21

Advise patients to carry antihistamines with them if they suffer from allergic reactions, particularly in cases where specific causative agents are unknown

Question 22

Other than antihistamines, what should a patient with a severe allergy or history of anaphylaxis carry with them?

Answer 22

Adrenaline auto-injection pen

Question 23

What counselling is required with antihistamines regarding drowsiness and impairment of function?

Answer 23

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