Sedating Antihistamine

Aka: Sedating Antihistamine, First Generation Antihistamine, Antihistamine, Brompheniramine, Dimetane

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II. Indications

  1. Allergic Rhinitis
  2. Antiemetic
  3. Anaphylaxis (adjunct)
  4. Allergic Reaction
  5. Acute Urticaria
  6. Chronic Urticaria
  7. Antitussive (Cough Suppression)
  8. Drug-Induced Parkinsonism
  9. Anti-pruritic
    1. Suppresses itching, and reduces lesions
    2. Does not completely eradicate
    3. Hydroxyzine (Atarax) is most potent Antihistamine for Pruritus
  10. Insomnia
    1. Not recommended (On Beers List, risk of Akathisia)

III. Mechanism

  1. See Histamine
  2. H1 Blockers
    1. First Generation Antihistamine have more potent Anticholinergic activity (sedation, Dry Mouth)
    2. Second Generation Antihistamines (e.g. Fexofenadine) are labeled as Non-Sedating Antihistamines
  3. H2 Blockers (e.g. Cimetidine) are also Antihistamines
    1. Although primarily active in suppressing acid secretion from the Stomach, H2 Blockers have some H1 activity
  4. Vertigo Management agents are also Antihistamines
    1. See Meclizine
    2. See Dimenhydrinate
  5. Phenothiazines are also Antihistamines
    1. Antiemetics (e.g. Promethazine)
    2. Antipsychotics

IV. Types: Antihistamines (H1 Antagonists)

  1. Ethanolamines (high Anticholinergic activity)
    1. Clemastine (Tavist)
    2. Carbinoxamine
    3. Dimenhydrinate (Dramamine)
    4. Diphenhydramine (Benadryl)
    5. Doxylamine (Unisom)
  2. Alkylamines (moderate Anticholinergic activity, low to moderate sedation)
    1. Brompheniramine (Dimetane, found mostly in combination products including Dimetapp)
    2. Chlorpheniramine (Chlor-Trimeton)
    3. Dexchlorpheniramine (Polaramine, Polmon)
  3. Phenothiazine: Piperidines (moderate Anticholinergic activity, low to moderate sedation)
    1. Cyproheptadine (Periactin)
    2. Loratadine (Claritin) and its active isomer Desloratadine (Clarinex)
    3. Fexofenadine (Allegra)
  4. Phenothiazine: Piperazines (moderate Anticholinergic activity, low to moderate sedation)
    1. Hydroxyzine (and its active metabolites, Cetirizine and Levocetirizine)
    2. Meclizine (Antivert)
  5. Intranasal Antihistamines
    1. Azelastine (Astelin) nasal spray
    2. Olopatadine (Patanol)
  6. Ocular Antihistamine
    1. See Ocular Antihistamine
    2. Naphazoline (Vasocon, Naphcon)
    3. Epinastine (Elestat)
    4. Bepotastine (Bepreve)
    5. Alcaftadine (Lastacraft)

V. Adverse Effects

  1. See Anticholinergic Toxicity
  2. Sedation, or, in children, paradoxical excitability
  3. Worsens Benign Prostatic Hyperplasia
  4. Worsens Glaucoma
  5. Dizziness
  6. Respiratory depression
  7. Tachycardia
  8. Seizures
  9. QTc Prolongation (and Torsades de Pointes)
    1. Especially with Hydroxyzine
  10. Anticholinergic Symptoms
    1. Delirium (elderly)
    2. Hallucinations
    3. Blurred Vision
    4. Agitation

VI. Medications: First Generation Antihistamines (Sedating Antihistamines)

  1. See Meclizine
  2. See Dimenhydrinate
  3. See Doxylamine
  4. Carbinoxamine (Palgic, Ryvent, pregnancy category C) - Regular Release Formulation
    1. Adult: 4 to 8 mg orally three to four times daily
    2. Child age 2 to 3 years: 2 mg orally three to four times daily
    3. Child age 3 to 6 years: 2 to 4 mg orally three to four times daily
    4. Child age 2 to 3 years: 4 to 6 mg orally three to four times daily
  5. Chlorpheniramine (Chlor-Trimeton, Pregnancy Category B, approved for age over 6 years)
    1. Dosing: Regular Release Formulation
      1. Adult (age >=12 years): 4 mg orally every 4 to 6 hours
      2. Child (age 6 to 11 years): 2 mg orally every 4 to 6 hours
      3. Child (age 2 to 5 years): 1 mg orally every 4 to 6 hours (NOT fda approved)
    2. Dosing: Timed Release Formulation
      1. Adult (age >=12 years): 8 mg orally every 8 hours OR 12 mg orally every 12 hours
      2. Child (age 6 to 11 years): 8 mg orally every 12 hours (NOT fda approved)
  6. Clemastine (Tavist)
    1. Dosing: Allergic Rhinitis
      1. Adult (age >=12 years): 1.34 mg orally twice daily (maximum 8.04 mg/day)
      2. Child (age 6 to 11 years): 0.67 mg orally twice daily (maximum 4.02 mg/day)
    2. Dosing: Urticaria or Angioedema
      1. Adult (age >=12 years): 2.68 mg orally two to three times daily (maximum 8.04 mg/day)
      2. Child (age 6 to 11 years): 1.34 mg orally twice daily (maximum 4.02 mg/day)
  7. Cyproheptadine (Periactin)
    1. Dosing: Allergic Rhinitis or Urticaria
      1. Adult (age >14 years): Start 4 mg orally three times daily (maximum 32 mg/day)
      2. Child (age 7 to 14 years): Start 4 mg orally three times daily (maximum 16 mg/day)
      3. Child (age 2 to 6 years): Start 2 mg orally three times daily
    2. Dosing: Serotonin Syndrome
      1. See Serotonin Syndrome
      2. Start 12 mg orally for first dose
      3. Next: 2 mg orally every 2 hours until symptoms resolve
      4. Next 8 mg every 6 hours until syndrome completes
  8. Diphenhydramine (Benadryl, Pregnancy Category B, approved for age over 6 years)
    1. Adult (age >=12 years): 25-50 mg PO/IV/IM every 6 hours (maximum: 300 to 400 mg/day)
    2. Child (age <12 years): 5 mg/kg/day divided four times daily (12.5 mg/5 ml)
      1. Age 2 to 5 years: 6.25 mg every 4 hours as needed
      2. Age 6 to 12 years: 12.5 mg every 4 years as needed
  9. Hydroxyzine (Atarax, Vistaril)
    1. Dosing: Pruritus
      1. Adult: 25 to 100 mg orally or IM three to four times daily as needed
      2. Child (age 4 to 6 years): 50 mg/day divided four times daily as needed
      3. Child (age >=6 years): 50 to 100 mg/day divided four times daily as needed
    2. Dosing: Anxiety
      1. Adult: 50 to 100 mg orally three to four times daily as needed

VII. Resources

  1. (2017) Antihistamines, LiverTox, Bethesda
    1. https://www.ncbi.nlm.nih.gov/books/NBK547896/

VIII. References

  1. Olson (2020) Clinical Pharmacology, Medmaster Miami, p. 146-7

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