What is the recommended dose and usage of diphenhydramine (antihistamine) for treating motion sickness?

Diphenhydramine for Motion Sickness: Dosing and Usage Guidelines

For motion sickness prevention and treatment, the recommended dose of diphenhydramine is 25-50 mg taken orally 30 minutes before travel, which can be repeated every 4-6 hours as needed, not exceeding 300 mg in 24 hours. This first-generation antihistamine is moderately effective for motion sickness but causes significant sedation as its primary side effect.

Efficacy and Evidence

Diphenhydramine has been established as a treatment option for motion sickness, though the evidence shows varying degrees of effectiveness:

• A Cochrane systematic review found that first-generation antihistamines like diphenhydramine are probably more effective than placebo at preventing motion sickness symptoms under natural conditions (40% vs 25% prevention rate) 1
• However, in direct comparison studies, diphenhydramine was not superior to placebo in treating established motion sickness during ambulance transport in mountainous terrain 2
• Scopolamine remains more effective than antihistamines for severe motion sickness prevention 3

Dosing Protocol

Adult Dosing:

• Prevention: 25-50 mg orally 30 minutes before anticipated motion exposure
• Treatment of existing symptoms: 25-50 mg orally
• Frequency: May repeat every 4-6 hours as needed
• Maximum daily dose: 300 mg in 24 hours

Pediatric Dosing:

• 1-2 mg/kg per dose (maximum 50 mg) 4, 5
• May repeat every 6 hours as needed

Administration Timing

For maximum effectiveness, diphenhydramine should be administered 30 minutes before exposure to motion. This allows the medication to reach therapeutic levels before motion sickness symptoms begin.

Side Effects and Precautions

Diphenhydramine has significant side effects that must be considered:

• Sedation: Most common side effect (66% vs 44% with placebo) 1
• Other common side effects include:
  • Dizziness
  • Dry mouth
  • Blurred vision
  • Urinary retention
  • Wheezing

Important Precautions:

• Avoid alcohol and other CNS depressants which can increase sedation
• Do not operate vehicles or machinery after taking diphenhydramine
• Use with caution in elderly patients who may be more sensitive to anticholinergic effects

Alternative Options

If diphenhydramine is not suitable or effective:

1. Scopolamine: Available as a transdermal patch (first-line for severe motion sickness)
2. Other first-generation antihistamines: Chlorpheniramine (4-12 mg) has shown efficacy with potentially less sedation 6
3. Non-pharmacological approaches:
   • Positioning in the most stable part of the vehicle
   • Focusing on the visual horizon
   • Gradual, intermittent exposure to motion

Clinical Decision Algorithm

1. Assess severity and setting:

   • Mild anticipated motion sickness → Diphenhydramine 25 mg
   • Moderate to severe anticipated motion sickness → Diphenhydramine 50 mg or consider scopolamine
   • Need to remain alert (e.g., drivers) → Avoid diphenhydramine, consider non-sedating alternatives

2. Consider patient factors:

   • Age (reduced dose for elderly)
   • Prior response to antihistamines
   • Concurrent medications that may interact

3. Timing:

   • Administer 30 minutes before travel
   • Plan dosing schedule for longer journeys

Common Pitfalls

• Underdosing: Using less than 25 mg may be ineffective for motion sickness
• Inappropriate timing: Taking diphenhydramine after symptoms begin is less effective
• Expecting complete prevention: Even at optimal dosing, diphenhydramine reduces but doesn't eliminate all motion sickness symptoms
• Using non-sedating antihistamines: Second-generation antihistamines (cetirizine, loratadine) are NOT effective for motion sickness 7

Diphenhydramine remains a reasonable option for motion sickness when sedation is acceptable, but patients should be clearly informed about its limitations and side effects before use.