Is it safe to take Benadryl (diphenhydramine) with dimenhydrinate?

Safety of Taking Benadryl (Diphenhydramine) with Dimenhydrinate

Taking Benadryl (diphenhydramine) with dimenhydrinate is not recommended due to potential additive anticholinergic effects and increased risk of toxicity, as dimenhydrinate already contains diphenhydramine as one of its components.

Understanding the Medications

• Dimenhydrinate is a combination drug consisting of diphenhydramine and 8-chlorotheophylline (a theophylline derivative) 1
• Taking Benadryl (diphenhydramine) alongside dimenhydrinate would effectively be doubling the diphenhydramine dose, increasing risk of adverse effects 2
• Both medications have significant anticholinergic and sedative properties that would be compounded when taken together 3

Potential Risks of Combination

• Increased risk of severe anticholinergic side effects including:

  • Excessive sedation and impaired cognitive function 3, 4
  • Urinary retention 5
  • Dry mouth, blurred vision, and constipation 3
  • Increased risk of hypotension 3
  • Potential for cardiac arrhythmias at high doses 6

• Risk of central nervous system toxicity:

  • At high doses, these medications can cause paradoxical excitation, hallucinations, seizures, and delirium 2
  • Fatal cases have been reported with massive overdoses of dimenhydrinate alone 6

Clinical Considerations

• Both medications are first-generation antihistamines with significant blood-brain barrier penetration 3
• The combination could potentially enhance the rewarding properties, increasing risk of misuse 7
• Acute tolerance may develop to some effects with repeated dosing, potentially leading to dose escalation 4
• The FDA label for dimenhydrinate specifically lists hypersensitivity to diphenhydramine as a contraindication 1

Alternative Approaches

• If antihistamine effects are needed:

  • Use only one of these medications at the recommended dose 2
  • Consider second-generation antihistamines with less sedation and fewer anticholinergic effects for allergic symptoms 3

• If seeking sleep aid effects:

  • The American Academy of Sleep Medicine does not recommend diphenhydramine for chronic insomnia 8
  • Non-pharmacological approaches like cognitive-behavioral therapy for insomnia (CBT-I) are preferred first-line treatments 5

Special Populations at Higher Risk

• Elderly patients are more sensitive to anticholinergic effects 3
• Patients with urinary retention, glaucoma, or cardiovascular disease 5
• Those taking other medications with sedative or anticholinergic properties 3

Emergency Management

• If someone has taken both medications and is experiencing severe symptoms (agitation, hallucinations, seizures, or respiratory depression), they should be referred to an emergency department immediately 2
• The toxic dose threshold for diphenhydramine is approximately 7.5 mg/kg or 300 mg (whichever is less) in patients 6 years and older 2