Dimenhydrinate
H1 antihistamines
H1 antihistamines
A sedating H1 antihistamine (a salt of diphenhydramine) used to prevent and treat the nausea, vomiting and vertigo of motion sickness.
Quick facts
- Group
- H1 antihistamines
- Target
- Histamine H1 receptor (and muscarinic receptors)
- Used for
- Prevention and treatment of nausea
- Key effects
- Drowsiness · Dizziness · Dry mouth
- Monitor
- Level of sedation · Urinary retention and confusion
- ATC
- R06AA11
No doses: the clinician sets them by the current guideline.
Mechanism of action
The figure shows doxepin, a first-generation H1 blocker, in the human H1 receptor; diphenhydramine, the active part of dimenhydrinate, binds the same site.
Dimenhydrinate is the 8-chlorotheophylline salt of diphenhydramine, in about equal parts. Diphenhydramine blocks central H1 receptors and muscarinic receptors in the vestibular pathways and the vomiting centre. This damps the overstimulated signal from the inner ear that causes motion sickness. The same central H1 and antimuscarinic actions explain the drowsiness and the dry mouth.
- Motion overstimulates the labyrinth and vestibular nuclei
- Diphenhydramine blocks central H1 and muscarinic receptors
- Vestibular input to the vomiting centre is damped
- Nausea, vomiting and vertigo ease; drowsiness is expected
Indications
- Prevention and treatment of nausea, vomiting and vertigo of motion sickness
Adverse effects
Common
- Drowsiness (most common)
- Dizziness, headache, nervousness and insomnia
- Dry mouth, nose and throat; blurred vision
- Difficulty passing urine
- Tachycardia, epigastric discomfort and rash
Contraindications
- Neonates (the injection contains benzyl alcohol, linked to the fatal gasping syndrome in premature infants)
- Hypersensitivity to dimenhydrinate, diphenhydramine or 8-chlorotheophylline
- Caution in conditions that anticholinergic drugs can worsen, such as narrow-angle glaucoma or prostatic enlargement
Key interactions
- Alcohol and other CNS depressants
- Additive sedation and impaired driving
- Certain antibiotics that can damage hearing
- Dimenhydrinate may mask early ototoxic symptoms until damage is irreversible
What is monitored
- Level of sedation, especially in older adults
- Urinary retention and confusion in older adults
- Hearing symptoms in patients on ototoxic antibiotics
Administration and safety
- The injection is given intramuscularly or, after dilution in sodium chloride, slowly intravenously
- Never injected into an artery
- Parenteral solution is inspected for particles and discolouration
- For motion sickness it is given before the journey
Pregnancy and breastfeeding
Human data do not show an increased risk of malformations, but it is used only if clearly needed; safety in labour is not established.
Nursing care
- Warn about drowsiness: no driving or machinery, and no alcohol
- Raise bed rails and help older adults walk after a dose
- Check for urinary retention and dry mouth
- Keep out of reach of children: overdose is dangerous in them
Pharmacokinetics
- Absorption
- Oral (tablets, chewable tablets, syrup) or injection (intramuscular or diluted intravenous)
- Onset and duration
- Used before travel to prevent symptoms, and also to treat them
Test yourself
Think of the answer, then tap the question to see it.
What is dimenhydrinate chemically?
The 8-chlorotheophylline salt of diphenhydramine.
What is the most common adverse effect of dimenhydrinate?
Drowsiness.
Why is dimenhydrinate injection contraindicated in neonates?
It contains benzyl alcohol, linked to the fatal gasping syndrome in premature infants.
Which risk arises when dimenhydrinate is given with ototoxic antibiotics?
It may mask early ototoxic symptoms until the damage is irreversible.
Connections
← Back to the class: H1 antihistamines
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - DIMENHYDRINATE injection, solution (DailyMed (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).
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Representative model, for educational purposes only.