Antihistamines Flashcards
What are some examples of first-generation antihistamines?
Diphenhydramine, Dimenhydrinate, Chlorpheniramine, Meclizine, Promethazine, Hydroxyzine.
What is the primary mechanism of first-generation antihistamines?
H1 receptor antagonists with additional effects on M1 (muscarinic), 5-HT (serotonin), and alpha-adrenergic receptors.
What are the clinical uses of first-generation antihistamines?
Used for allergy, motion sickness, and insomnia.
What are the major side effects of first-generation antihistamines?
Sedation, antimuscarinic effects (urinary retention, dry mouth, constipation, confusion in elderly), anti-alpha effects (postural hypotension), and anti-serotonergic effects (increased appetite, weight gain).
Why do first-generation antihistamines cause sedation?
They cross the blood-brain barrier and antagonize central H1 receptors, leading to drowsiness.
Why should first-generation antihistamines be avoided in the elderly?
They cause confusion, urinary retention, and sedation, which increases fall risk.
Which first-generation antihistamine is commonly used for motion sickness?
Meclizine and Dimenhydrinate due to their strong anti-muscarinic effects.
Which first-generation antihistamine is used for nausea and vomiting?
Promethazine due to its anti-dopaminergic and anti-histaminic effects.
What are the effects of first-generation antihistamines on alpha-adrenergic receptors?
They block alpha receptors, leading to postural hypotension.
What are the effects of first-generation antihistamines on serotonin receptors?
They block 5-HT receptors, leading to increased appetite and weight gain.
Do first-generation antihistamines affect the P450 system?
No significant interactions with the P450 enzyme system.
What are some examples of second-generation antihistamines?
Loratadine, Fexofenadine, Desloratadine, Cetirizine.
How do second-generation antihistamines differ from first-generation?
They are more selective for peripheral H1 receptors and do not significantly cross the blood-brain barrier, reducing sedation.
What are the clinical uses of second-generation antihistamines?
Used for allergic rhinitis and urticaria.
What is the advantage of second-generation antihistamines over first-generation?
Less sedation due to minimal penetration of the blood-brain barrier.
Which second-generation antihistamine has the highest risk of sedation?
Cetirizine can cause mild drowsiness in some individuals.
Are second-generation antihistamines associated with significant anti-muscarinic effects?
No, they have minimal to no anti-muscarinic effects.
Which second-generation antihistamine is best for patients who need to avoid sedation?
Fexofenadine has the least sedative effect.
Do second-generation antihistamines cause weight gain?
No, they do not significantly affect serotonin receptors.
Are second-generation antihistamines metabolized by the P450 system?
Loratadine and Desloratadine are metabolized by CYP3A4, while Fexofenadine is not significantly metabolized by CYP enzymes.
Which second-generation antihistamine is preferred for renal impairment?
Fexofenadine is preferred as it has minimal metabolism and is eliminated unchanged in urine.
