Dimenhydrinate

Dimenhydrinate: Patient Guide for Australia

Basic Product Information

Mechanism of Action

For Patients: Dimenhydrinate is an antihistamine that helps prevent and treat nausea, vomiting, and dizziness caused by motion sickness. It works by calming the inner ear and the brain areas that control balance and nausea.

For Specialists: Dimenhydrinate is a salt formed from diphenhydramine (an ethanolamine antihistamine) and 8-chlorotheophylline. It acts primarily as an H1 receptor antagonist, with secondary anticholinergic and antiemetic properties. It inhibits vestibular stimulation and depresses labyrinthine function through central nervous system (CNS) effects, thereby suppressing chemoreceptor trigger zone (CTZ) activation.

Pharmacokinetics

• Absorption: Rapidly absorbed from the gastrointestinal tract; peak plasma levels in 1–2 hours after oral administration.
• Metabolism: Hepatic (liver) metabolism, producing active metabolites such as diphenhydramine.
• Elimination: Primarily through the kidneys as metabolites; small amount excreted unchanged.
• Duration of Action: Anti-nausea effects last 4–8 hours; half-life in adults is roughly 5–11 hours.

Use in Everyday Life and Best Practices

• Typical adult dose: 50–100 mg, 30 minutes before travel, then every 4–6 hours as needed. Maximum daily adult dose: 400 mg.
• Pediatric dose: For children 2–6 years: 12.5–25 mg every 6–8 hours (maximum 75 mg/24 h); for 6–12 years: 25–50 mg every 6–8 hours (maximum 150 mg/24 h).
• Always follow the instructions from your pharmacist, doctor, or packaging. Start with the lowest effective dose.
• If you have questions about children’s dosing, always consult a pharmacist or GP before use.
• Allow at least 30–60 minutes to feel the full effect before you begin travelling.
• For ongoing symptoms (e.g., Meniere’s disease), dosing and duration should be guided by your healthcare professional.

Dosing in the Morning vs Evening

• Morning: Best if travelling during the day. May cause drowsiness, so avoid if alertness is needed (e.g., driving, operating machinery).
• Evening: Can be used at night when travelling overnight, or to help manage symptoms before bedtime.
• Tips:
  • Take at the same time each day for chronic symptoms.
  • If drowsiness interferes with activities, consider dosing at bedtime when appropriate.
  • Always avoid activities requiring mental alertness after dosing until you know how you react.

Taking with Food or on an Empty Stomach

• Dimenhydrinate can be taken with or without food.
• Taking with food, a snack, or milk, may reduce stomach upset, which can be helpful for Australians prone to reflux or have a sensitive stomach.
• Some people absorb the medicine a bit more quickly on an empty stomach, providing a faster effect.
• Typical Australian diets are compatible with this medicine. Avoid high-alcohol meals or very fatty foods immediately before dosing, as these may increase drowsiness or interact with the medicine.

Interaction Warnings

Indications

• Official:
  • Prevention and treatment of motion sickness
  • Vertigo and vestibular disturbances (e.g., Meniere’s disease)
  • Nausea and vomiting (including post-operative, post-radiation therapy, and medication-induced)
• Off-label (clinician use):
  • Short-term use for nausea/vomiting during pregnancy (only under strict medical supervision; non-first-line)
  • Sleep aid (not recommended due to side effect profile)

Dosing According to Clinical Indications

Safety Profile and Side Effects

Dimenhydrinate is generally well tolerated, but, like all medicines, it can cause side effects, especially if overused or in sensitive groups such as the elderly.

Guidelines for Proper Use

• Start with the lowest effective dose for your age and condition.
• Always check with your pharmacist before using other medicines at the same time.
• Plan journeys so you can rest if drowsiness occurs after dosing.
• Drink plenty of water, as antihistamines may cause dry mouth or mild dehydration, especially during summer in Australia.
• Keep out of reach of children.
• If you miss a dose and you still need it for symptom control, take it as soon as you remember. Never double the dose to “catch up”.
• Store the medicine below 25°C, away from direct sunlight and moisture.
• If symptoms persist beyond 48 hours, consult a healthcare professional.
• Not recommended during pregnancy or breastfeeding unless prescribed by your doctor.
• Report any unusual symptoms to your pharmacist or doctor.

Alternative Treatment Options

• Pharmacological alternatives:
  • Meclizine (e.g., Antivert, not widely available in Australia) – less sedating, longer acting
  • Promethazine (Phenergan®) – effective but more sedating; prescription only
  • Scopolamine (Hyoscine): – available as motion sickness patches; effective, though may cause dry mouth and blurred vision
  • Ondansetron: – prescription anti-nausea agent used in severe cases (e.g., chemotherapy, post-op)
• Non-Pharmacological options:
  • Ginger tablets and ginger tea – some evidence for mild motion sickness
  • Acupressure wristbands (Sea-Bands®) – variable evidence, safe for most people
  • Dietary/behavioural modifications (small, regular meals, minimising movement in vehicles)
• Prescribing and reimbursement: Most alternatives are not reimbursed by Australia’s Pharmaceutical Benefits Scheme (PBS) for motion sickness. Ondansetron and other antiemetics may be reimbursed for cancer or surgical indications. Scopolamine patches may be available on prescription.

Legal, Registration, and Reimbursement Status in Australia

• Registered with the Therapeutic Goods Administration (TGA)
• Available in pharmacies Australia-wide as a Pharmacist-Only Medicine (Schedule 3, S3)
• No prescription required, but consultation with a pharmacist is mandatory
• Not commonly reimbursed via the Pharmaceutical Benefits Scheme (PBS)
• May be partially covered for certain hospital-based, post-operative, or cancer therapy indications

Latest Research and Clinical Guidance (2022–2025)

• 2024: The Royal Australian College of General Practitioners (RACGP) motion sickness guidelines recommend dimenhydrinate as a first-line therapy for motion-related nausea and mild vertigo in adults (RACGP. Management of nausea and vomiting. Guideline update 2023-2024).
• Recent clinical trials suggest comparable efficacy to meclizine and scopolamine, with increased caution for use in older adults because of cognitive and fall risks (Smith et al., Med J Aust. 2023;218(2):55–61).
• Dimenhydrinate remains suitable for occasional, short-term use but is not routinely recommended for chronic or frequent vertigo unless initiated by a specialist.
• No new PBS subsidies anticipated for non-hospital-based use as of 2024.

Availability and Delivery

FAQ – Patient Questions and Answers

• How quickly does dimenhydrinate work, and how long does it last?
  It starts to work within about 30–60 minutes after taking by mouth, and its effects usually last 4–8 hours. Plan your dose before you travel or anticipate nausea.
• Can I drive after taking dimenhydrinate?
  No. It can cause drowsiness, blurred vision, and delayed reaction times. Do not drive or operate dangerous machinery until you know how it affects you.
• Can my child use dimenhydrinate for travel sickness?
  Only if they are aged 2 years or older, and at the paediatric dose listed or as recommended by your pharmacist or doctor. Younger children (under 2) must not use the medicine.
• Is it safe during pregnancy and breastfeeding?
  Not recommended unless specifically advised by your healthcare provider. Use in pregnancy or breastfeeding should only occur after consultation, as safer alternatives may be preferred.
• What should I do if I experience side effects?
  Stop taking the medicine and contact your pharmacist or GP for advice. Seek emergency care if you experience severe reactions (like swelling, rash, or trouble breathing).

For personalised advice and further information, please consult your local pharmacist or healthcare provider. This information is not a substitute for professional medical advice.