Promethazine

Sounds like 'pro-meth-ah-zeen'

Easy-to-read medicine information about promethazine – what it is, how to take it safely and possible side effects. Promethazine is commonly called Phenergan or Allersoothe.

Type of medicine Also called
  • Belongs to groups of medicines called antihistamines
  • Allersoothe®
  • Phenergan®

What is promethazine?

Promethazine is an antihistamine that has a number of uses:

  • To relieve allergic conditions such as hay fever and skin rashes.
  • To prevent nausea (feeling sick) or vomiting (being sick), including when caused by motion sickness. It works by blocking certain chemicals (histamines) in your brain that can cause nausea and vomiting.
  • To make you feel sleepy. It may be taken for a few days only to help promote sleep in adults with sleeping problems.  

Promethazine is available as an injection, a liquid or tablets (10 mg and 25 mg). In New Zealand, promethazine liquid and tablets can be bought from a pharmacy without a prescription, after consultation with a pharmacist (pharmacist-only medicine). 

Promethazine must not be used for sedation of children (ie, to make children sleep. It can cause severe breathing problems).

Dose

Promethazine tablets are available in 2 strengths: 10 mg and 25 mg. The dose of promethazine depends on its use. Your doctor or pharmacist will advise you on the best dose for you. The following is a guide. 

  • Allergies: One to three 25mg tablets taken as a single dose at night, or one to two 10mg tablets taken 2 to 3 times daily. 
  • Motion sickness: 25mg taken the night before travel and repeated after 6 to 8 hours on the following day if needed. 
  • Sedation: One to three 25mg tablets taken as a single dose at night.

How to use promethazine

  • Take promethazine with a glass of water.
  • You can take it with or without food.
  • Limit drinking alcohol while you are taking promethazine. Alcohol can increase the risk of side effects such as drowsiness.
  • Protect yourself from the sun (with sunblock, a hat and long sleeves). Promethazine can make you more sensitive to the sun.
  • It is not harmful if you miss a dose of promethazine. If you forget your dose, take it as soon as you remember. But if it is nearly time for your next dose, just take the next dose at the right time. Do not take double the dose.    

Motion sickness: If you are taking promethazine to prevent travel sickness, it is usually recommended that you take the first dose at bedtime on the evening before you are due to travel. You can then take a further dose on the morning of your travel if needed. Promethazine causes drowsiness or sleepiness and is not recommended if you need to stay alert such as if you are driving.

Precautions – before starting promethazine

  • Are you pregnant or breastfeeding?
  • Do you have problems with your liver or kidneys?
  • Do you have a heart condition?
  • Do you have glaucoma (increased pressure in your eye)?
  • Do you have epilepsy?
  • Do you have problems passing urine (peeing) or prostate problems?
  • Are taking any other medicines? This includes any medicines you can buy without a prescription, as well as herbal and complementary medicines.

If so, it’s important that you tell your doctor or pharmacist before you start promethazine. Sometimes a medicine isn’t suitable for a person with certain conditions, or it can only be used with extra care.

Side effects

Like all medicines, promethazine can cause side effects, although not everyone gets them. Often side effects go away once your body gets used to the new medicine.

Side effects What should I do?
  • Drowsiness
  • Dizziness
  • Be careful when driving or using tools until you know how this medicine affects you.
  • These effects put you at risk of falls and injuries especially if you are elderly. Tell your doctor if you are concerned.
  • Do not drink alcohol.
  • Dry mouth 
  • Experiencing a dry mouth is common when taking promethazine.
  • Try sucking on ice chips or lollies, or drink small sips of water.
  • Read more about dry mouth.
  • Sudden changes in mood
  • See your doctor.
  • Blurred vision or fast heart rate
  • See your doctor.
  • Constipation (hard poos)
  • Eat a high fibre diet that includes a lot of fruit, vegetables, brown bread and bran-based breakfast cereals.
  • Drink plenty of water.
  • Talk to your doctor if you have discomfort.
  • Read more about constipation.

Interactions

Promethazine can interact with some medicines and herbal supplements so check with your doctor or pharmacist before starting promethazine and before starting any new medicines.

Learn more

The following links have more information on promethazine:

Promethazine NZ Formulary Patient Information

References

  1. Promethazine NZ Formulary
  2. Do not use oral sedating antihistamines for sedation of children Medsafe, NZ, 2020
Credits: Sandra Ponen, Pharmacist. Reviewed By: Angela Lambie, Pharmacist, Auckland Last reviewed: 10 Feb 2021