First-Generation Antihistamines

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Questions and Answers

A patient taking a first-generation antihistamine reports experiencing significant drowsiness and cognitive impairment. What pharmacological property of these drugs is most directly responsible for these side effects?

  • High lipophilicity, enabling them to cross the blood-brain barrier and interact with CNS H1 receptors. (correct)
  • Agonistic activity at muscarinic receptors, causing downstream effects on neuronal excitability.
  • Inhibition of histamine release from mast cells, reducing overall allergic response.
  • Selective antagonism of peripheral H1 receptors, preventing histamine-mediated vasodilation.

Which of the following best describes the mechanism by which first-generation antihistamines alleviate motion sickness?

  • Through their central anticholinergic effects, influencing the vestibular nuclei and chemoreceptor trigger zone (CTZ). (correct)
  • By peripherally blocking histamine-induced vasodilation, preventing hypotensive responses that trigger nausea.
  • By antagonizing dopamine receptors in the area postrema, reducing emetic signals.
  • By selectively blocking H2 receptors in the gastrointestinal tract, reducing acid secretion and nausea.

A geriatric patient is prescribed diphenhydramine for insomnia. Which of the following potential side effects is of greatest concern in this population?

  • Increased appetite and weight gain.
  • Paradoxical excitation and hyperactivity.
  • Urinary retention, confusion, and increased risk of falls. (correct)
  • Tachycardia and hypertension.

A patient with both seasonal allergies and benign prostatic hyperplasia (BPH) seeks an over-the-counter antihistamine. Which first-generation antihistamine should be avoided?

<p>Diphenhydramine, because its anticholinergic effects can exacerbate urinary retention in BPH. (B)</p> Signup and view all the answers

A patient is experiencing serotonin syndrome due to medication interactions. Which first-generation antihistamine could potentially be used to manage this condition?

<p>Cyproheptadine, which possesses antihistaminic and antiserotonergic effects. (A)</p> Signup and view all the answers

Why are first-generation antihistamines generally avoided in patients with narrow-angle glaucoma?

<p>Their anticholinergic properties can cause mydriasis, potentially precipitating acute angle closure. (B)</p> Signup and view all the answers

A child is given promethazine for nausea, and develops muscle rigidity, fever, and altered mental status. What potentially life-threatening condition should be suspected?

<p>Neuroleptic malignant syndrome (NMS), a rare but serious reaction to dopamine-blocking drugs like promethazine. (A)</p> Signup and view all the answers

A researcher is investigating the binding properties of first-generation antihistamines. Which statement accurately describes their interaction with histamine receptors?

<p>They act as inverse agonists, stabilizing the inactive state of the histamine receptor and reducing baseline activity. (A)</p> Signup and view all the answers

In which clinical scenario would the appetite-stimulating effect of cyproheptadine be most beneficial?

<p>Management of anorexia nervosa or cachexia. (A)</p> Signup and view all the answers

A pregnant woman is experiencing severe nausea and vomiting. While some antihistamines are used cautiously, which specific combination is sometimes considered relatively safer in pregnancy for managing these symptoms?

<p>Doxylamine combined with pyridoxine (vitamin B6). (B)</p> Signup and view all the answers

Which class of medications would have synergistic effects with first-generation antihistamines, increasing the risk of anticholinergic side effects?

<p>Tricyclic antidepressants (TCAs). (A)</p> Signup and view all the answers

What is the primary reason first-generation antihistamines are more likely to cause drowsiness compared to second-generation antihistamines?

<p>First-generation antihistamines readily cross the blood-brain barrier, while second-generation antihistamines do not. (B)</p> Signup and view all the answers

A patient taking an MAOI begins using diphenhydramine for sleep. What potential interaction should the patient be warned about?

<p>Enhanced anticholinergic and serotonergic effects. (B)</p> Signup and view all the answers

A patient with COPD is prescribed an antihistamine. What is the primary concern regarding the use of first-generation antihistamines in this patient population?

<p>Thickening of respiratory secretions, potentially worsening breathing. (D)</p> Signup and view all the answers

Which of the following is the most accurate description of the antiemetic activity of first-generation antihistamines?

<p>Influence on the vestibular nuclei and chemoreceptor trigger zone (CTZ) in the brain. (C)</p> Signup and view all the answers

A patient presents with dry mouth, blurred vision, and urinary retention after starting an over-the-counter medication. Which receptor type is most likely responsible for these effects?

<p>Muscarinic acetylcholine receptors. (B)</p> Signup and view all the answers

Which first-generation antihistamine carries the highest risk of extrapyramidal symptoms?

<p>Promethazine. (A)</p> Signup and view all the answers

A patient with a known allergy to diphenhydramine requests an alternative medication for motion sickness. Which of the following would be the LEAST appropriate choice?

<p>Dimenhydrinate. (B)</p> Signup and view all the answers

How do first-generation antihistamines contribute to local anesthetic effects?

<p>By directly blocking voltage-gated sodium channels. (B)</p> Signup and view all the answers

Why is it important to avoid first-generation antihistamines in nursing mothers?

<p>They can be excreted in breast milk, potentially causing sedation or anticholinergic effects in the infant. (C)</p> Signup and view all the answers

Flashcards

Common 1st Generation Antihistamines

Diphenhydramine, Chlorpheniramine, Hydroxyzine, Promethazine, Meclizine, Dimenhydrinate, Doxylamine, Cyproheptadine

Mechanism of Action of 1st Generation Antihistamines

Block histamine H1 receptors in the peripheral and central nervous system (CNS).

Indications for 1st Generation Antihistamines

Allergic reactions, anaphylaxis (adjunct), motion sickness, insomnia, anxiety, vertigo, parkinsonism, appetite stimulation.

Common Side Effects of 1st Generation Antihistamines

Sedation, dry mouth/eyes, blurred vision, constipation, urinary retention, dizziness, confusion.

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Specific Side Effect of Promethazine

Risk of extrapyramidal symptoms, neuroleptic malignant syndrome, and severe tissue injury if given IV.

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Specific effect of Hydroxyzine

Strong anxiolytic effect

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Specific action of Cyproheptadine

Also blocks serotonin receptors—used for serotonin syndrome.

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Contraindications for 1st Generation Antihistamines

Elderly patients, glaucoma, BPH, asthma/COPD, pregnancy (caution), MAOI use.

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Binding of 1st Generation Antihistamines

They bind to histamine H1 receptors on smooth muscle, endothelium, and in the CNS.

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Inverse Agonist Action

They stabilize the inactive form of the H1 receptor, reducing baseline receptor activity.

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CNS Penetration

Due to their lipophilic structure, they easily cross the blood-brain barrier, resulting in sedation, drowsiness, and potential cognitive slowing.

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Anticholinergic Activity

Many 1st gen antihistamines also block muscarinic (M1) receptors, which causes dry mouth, urinary retention, tachycardia, and anti-nausea actions

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Study Notes

  • First-generation antihistamines include diphenhydramine, chlorpheniramine, hydroxyzine, promethazine, meclizine, dimenhydrinate, doxylamine, and cyproheptadine.
  • A mnemonic to remember them is "Don’t Call Her Pretty, Make Dinner During Class" (Diphenhydramine, Chlorpheniramine, Hydroxyzine, Promethazine, Meclizine, Dimenhydrinate, Doxylamine, Cyproheptadine).

Mechanism of Action

  • First-generation antihistamines are H1 receptor antagonists (inverse agonists).
  • They block histamine H1 receptors in both the peripheral and central nervous systems.
  • These antihistamines cross the blood-brain barrier, leading to sedative effects.
  • They possess anticholinergic, antiemetic, and local anesthetic properties due to nonspecific binding.
  • They bind to histamine H1 receptors on smooth muscle, endothelium, and in the CNS.
  • These drugs stabilize the inactive form of the H1 receptor, reducing its baseline activity.
  • Due to their lipophilic structure, these antihistamines easily cross the blood-brain barrier, causing sedation, drowsiness, and potential cognitive slowing and enhances their antiemetic and anti-motion sickness effects by acting on vestibular nuclei and the chemoreceptor trigger zone (CTZ).
  • They also block muscarinic (M1) receptors, leading to effects like dry mouth, urinary retention, tachycardia, and anti-nausea actions.

Indications

  • Used for allergic reactions like rhinitis, urticaria, and hay fever.
  • Used as an adjunct treatment for anaphylaxis.
  • Meclizine and dimenhydrinate are used for motion sickness and nausea.
  • Diphenhydramine and doxylamine are used for insomnia.
  • Hydroxyzine is used for anxiety and sedation.
  • Also used for vertigo.
  • Diphenhydramine can be used for parkinsonism and extrapyramidal symptoms.
  • Cyproheptadine is used for appetite stimulation.

Side Effects

  • Sedation and drowsiness
  • Dry mouth and dry eyes
  • Blurred vision
  • Constipation and urinary retention
  • Dizziness
  • Confusion, especially in the elderly
  • Paradoxical excitation in children
  • Promethazine carries a risk of extrapyramidal symptoms, neuroleptic malignant syndrome, and severe tissue injury if given IV.
  • Hydroxyzine has a strong anxiolytic effect.
  • Cyproheptadine also blocks serotonin receptors and is used for serotonin syndrome.

Contraindications

  • Should be avoided or used with caution in elderly patients due to increased risk of falls and delirium.
  • Use with caution in patients with glaucoma, especially narrow-angle.
  • Use with caution in patients with BPH because of urinary retention risk.
  • Use with caution in patients with asthma/COPD as drying of secretions can worsen symptoms.
  • During pregnancy, some agents like doxylamine + B6 are safe, while others like promethazine need caution.
  • Avoid in patients using MAOIs due to the risk of enhanced anticholinergic/serotonergic effects.

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