H1 Antihistamines

Question 1

A patient’s urticaria is interfering with sleep. Which class of drugs would be most appropriate to take before bed?

Answer 1

A first-generation antihistamine (e.g., promethazine). A first-generation antihistamine will cross the blood-brain barrier (BBB) and cause sedation helping the patient to sleep.

Question 2

Which class of antihistamine drugs would be most appropriate for a patient with nausea and vomiting?

Answer 2

A first-generation antihistamine. A first-generation antihistamine will cross the blood-brain barrier (BBB) and block histamine H1 and muscarinic M1 receptors to control nausea and vomiting.

Question 3

A 28-year-old taxi driver presents with allergic rhinitis. Suggest an appropriate antihistamine drug.

Answer 3

Fexofenadine, desloratadine or loratadine. Any second or new generation antihistamine that does not cause sedation would be appropriate.

Question 4

A 73-year-old patient with mild cognitive impairment presents with urticaria. Suggest an appropriate antihistamine drug?

Answer 4

Fexofenadine, desloratadine or loratadine. Any second or new generation antihistamine that does not cross the blood-brain barrier (BBB) to cause sedation and block muscarinic cholinergic receptors in the CNS.

Question 5

Use of promethazine is contraindicated in children under the age of 2 years old because of increased risk of…

Answer 5

Respiratory depression

Question 6

For how long should patients be warned not to drink fruit juice after taking fexofenadine?

Answer 6

4 hours before dosing and 1-2 hours after dosing

Question 7

List THREE drugs that should be avoided or taken only with caution together with all first-generation antihistamines.

Answer 7

Alcohol, opioid analgesics, antidepressants and any other CNS depressant drugs

Question 8

Which class of antihistamine would be preferable for the treatment of
urticaria in a patient with narrow-angle glaucoma?

Answer 8

Second or third/new generation antihistamines without muscarinic receptor antagonism

Question 9

What is the duration of action of MOST 2nd generation antihistamines?

Answer 9

12 to 24 hours

Question 10

What is the duration of action of MOST 1st generation antihistamines?

Answer 10

4 to 6 hours

Question 11

First-generation antihistamines can increase the risk of drug-induced long QT syndrome (LQTS) because of actions at…

Answer 11

HERG potassium channels

Question 12

What are the major histamine receptors on mast cells?

Answer 12

H1 and H4

Question 13

Why do 1st generation antihistamines cause sedation?

Answer 13

They cross the blood-brain barrier and inhibit H1 histamine receptors in the CNS. Inhibition of muscarinic receptors in the CNS may additionally further contribute to drowsiness/sedation.

Question 14

The triple vascular response of Lewis includes

Answer 14

Flush, Flare and Wheal

Question 15

Histamine triggers the formation of a wheal (a raised swelling on the skin) due to increased…

Answer 15

Vascular permeability

Question 16

Histamine triggers the formation of a flush due to…

Answer 16

Dilation of blood vessels and disruption of capillaries

Question 17

Allergens trigger mast cell degranulation by…

Answer 17

Cross-linking FcεR1-bound IgEs

Question 18

List at least THREE examples of 1st generation antihistamines

Answer 18

Chlorpheniramine, Triprolidine, Ketotifen, Diphenhydramine, Promethazine

Question 19

List at least THREE examples of 2nd generation antihistamines

Answer 19

Cetirizine, Loratadine, Fexofenadine

Question 20

List at least TWO examples of drugs sometimes considered 3rd or new generation antihistamines

Answer 20

Levocetirizine, Desloratadine

Question 21

Which histamine receptors on c-afferents and in the CNS contribute to itch?

Answer 21

H1 and H4

Question 22

In terms of agonist or antagonist activity, what kind of drugs are clinically used as antihistamine drugs?

Answer 22

Inverse agonists

Question 23

Which second-generation antihistamine is most likely to still cause some sedation?

Answer 23

Cetirizine (and also acrivastine) are less sedating that first-generation antihistamines but still notably more sedating than fexofenadine and loratadine.

Question 24

Why do 2nd generation antihistamines cross the blood-brain barrier (BBB) less?

Answer 24

2nd generation antihistamines are less lipophilic and/or have greater affinity for the P‐glycoprotein efflux pump than 1st generation antihistamines

Question 25

List at least THREE adverse effects of first-generation antihistamines

Answer 25

Sedation, dry mouth, GIT disturbances, weight gain, tachycardia, postural hypotension