Dimenhydrinate - Drug Monograph

Introduction

Dimenhydrinate is an over-the-counter and prescription antihistamine medication primarily used for the prevention and treatment of nausea, vomiting, and dizziness associated with motion sickness. It is a salt composed of diphenhydramine and 8-chlorotheophylline, with the former providing the primary therapeutic effects. First approved by the FDA in the 1940s, dimenhydrinate remains a widely used antiemetic agent in both clinical and self-care settings.

Mechanism of Action

Dimenhydrinate exerts its therapeutic effects primarily through competitive antagonism of histamine H₁ receptors in the central nervous system, particularly in the vestibular apparatus and vomiting center. The drug also possesses anticholinergic properties that contribute to its antiemetic effects by inhibiting stimulation of the vestibular system. The 8-chlorotheophylline component provides mild stimulant effects that may help counteract sedation caused by the diphenhydramine component.

Indications

• Prevention and treatment of nausea, vomiting, and dizziness associated with motion sickness
• Management of nausea and vomiting associated with other conditions (off-label)
• Vertigo management associated with vestibular pathologies (off-label)

Dosage and Administration

• Motion sickness prevention: 50-100 mg every 4-6 hours (maximum 400 mg/day)
• Treatment: 50-100 mg every 4-6 hours as needed

• Children 6-12 years: 25-50 mg every 6-8 hours (maximum 150 mg/day)
• Children 2-6 years: 12.5-25 mg every 6-8 hours (maximum 75 mg/day)
• Not recommended for children under 2 years

• Oral tablets: Administer with or without food
• Chewable tablets: Chew thoroughly before swallowing
• Liquid formulation: Use measuring device for accurate dosing
• For motion sickness prevention: Administer 30-60 minutes before anticipated travel

• Geriatric patients: Use lower doses due to increased sensitivity
• Hepatic impairment: Use with caution; consider dose reduction
• Renal impairment: Use with caution; consider dose reduction

Pharmacokinetics

Contraindications

• Hypersensitivity to dimenhydrinate, diphenhydramine, or other ethanolamine-derivative antihistamines
• Neonates and premature infants
• Patients with narrow-angle glaucoma
• Patients with bladder neck obstruction or prostatic hypertrophy
• Patients with stenosing peptic ulcer or pyloroduodenal obstruction
• Concurrent use with monoamine oxidase inhibitors (MAOIs)

Warnings and Precautions

• CNS effects: May cause drowsiness, sedation, and impaired cognitive function. Caution when operating machinery or driving.
• Anticholinergic effects: Use with caution in patients with cardiovascular disease, hypertension, hyperthyroidism, or asthma.
• Geriatric patients: Increased risk of falls, confusion, and anticholinergic effects.
• Pregnancy: Category B - Use only if clearly needed (limited human data)
• Lactation: Excreted in breast milk; use with caution
• Prolonged use: May lead to tolerance and decreased efficacy
• Risk of abuse: Potential for misuse and dependence with long-term use

Drug Interactions

• CNS depressants: Enhanced sedative effects with alcohol, benzodiazepines, opioids, and other sedating medications
• MAO inhibitors: Increased anticholinergic and sedative effects (contraindicated)
• Anticholinergic agents: Additive anticholinergic effects with tricyclic antidepressants, antipsychotics, and other anticholinergics
• CYP2D6 inhibitors: May increase dimenhydrinate levels (e.g., fluoxetine, paroxetine)
• Warfarin: Possible enhanced anticoagulant effect (monitor INR)

Adverse Effects

• Drowsiness/sedation
• Dry mouth
• Headache
• Dizziness
• Gastrointestinal discomfort
• Blurred vision

• Tachycardia
• Palpitations
• Hypotension
• Urinary retention
• Constipation
• Paradoxical excitation (especially in children)
• Allergic reactions (rash, urticaria)

• Anaphylaxis
• Severe hypotension
• Cardiac arrhythmias
• Seizures
• Acute angle-closure glaucoma
• Bone marrow suppression

Monitoring Parameters

• Efficacy: Reduction in nausea/vomiting symptoms
• Adverse effects: Sedation, anticholinergic effects
• Vital signs: Blood pressure, heart rate (especially in elderly)
• Mental status: Cognitive function, alertness
• Vision: Blurred vision, eye pain (glaucoma screening)
• Urinary function: Retention symptoms
• Fall risk assessment in elderly patients

Patient Education

• Take 30-60 minutes before travel for motion sickness prevention
• Avoid alcohol and other CNS depressants during treatment
• Be aware of potential drowsiness - avoid driving or operating machinery
• Use caution when rising from sitting/lying position due to dizziness risk
• Report severe drowsiness, difficulty urinating, vision changes, or palpitations
• Chewable tablets must be chewed thoroughly before swallowing
• Liquid formulation should be measured with proper device
• Do not exceed recommended dosage
• Store at room temperature away from moisture and heat
• Keep out of reach of children

References

1. Pharmaceuticals MR. Dramamine (dimenhydrinate) prescribing information. 2023. 2. Simons FE, Simons KJ. H1 antihistamines: current status and future directions. World Allergy Organ J. 2008;1(9):145-155. 3. Weiser M. Motion sickness: prevention and treatment. UpToDate. 2023. 4. Takeda N, Morita M, Hasegawa S, Horii A, Kubo T, Matsunaga T. Neuropharmacology of motion sickness and emesis. Acta Otolaryngol Suppl. 1993;501:10-15. 5. American Geriatrics Society Beers Criteria Update Expert Panel. American Geriatrics Society 2023 updated AGS Beers Criteria for potentially inappropriate medication use in older adults. J Am Geriatr Soc. 2023;71(7):2052-2081. 6. Lexicomp Online. Dimenhydrinate: drug information. Wolters Kluwer Clinical Drug Information. 2023. 7. Plushner SL. OTC agents for the treatment of motion sickness. US Pharm. 2008;33(6):26-29. 8. Spinks AB, Wasiak J, Villanueva EV, Bernath V. Scopolamine for preventing and treating motion sickness. Cochrane Database Syst Rev. 2007;(3):CD002851.