Promethazine Side Effects: What to Know

Medically approved by
Dr Earim Chaudry
Last updated
18th November 2021

In 30 seconds…

Promethazine is a medicine used to manage allergies and treat short-term sleep problems like insomnia. Luckily, promethazine side effects are not very common, but they can include daytime sleepiness, headaches, dizziness, and strange dreams or nightmares. To keep the risk low, never take more than the prescribed or recommended dose. 

Promethazine: One of the World’s Most Popular Sleep Medicines

Promethazine is the active ingredient in brands such as Night Nurse and Day & Night Nurse and is one of the world’s most popular medical sleep aids. It’s also very powerful. So, if you’re taking promethazine, it’s important you know what you are doing — and the possible risks.

That’s why we’re talking about promethazine side effects. While they are not common, they can have an impact on your day-to-day life.

What do you need to know about the side effects of promethazine? Let’s take a look.

What is Promethazine?

Promethazine is a type of drowsy antihistamine in a group of drugs known as phenothiazines. In simpler terms, it’s a drug that primarily combats the symptoms of allergies. However, it tends to make you quite sleepy. In fact, it’s more likely to cause drowsiness than many other allergy medications. And that’s why it’s a common ingredient in sleep aids.

Promethazine is usually prescribed and used for four main purposes:

  • Treating allergies such as hay fever, hives, or symptoms such as itchiness, a runny nose, or irritated eyes.
  • Aiding sleep problems such as insomnia or difficulty falling asleep — particularly when this is caused by itchiness or coughing.
  • Nausea and vomiting. That could be due to motion sickness or morning sickness.
  • Symptoms of a cold, like coughing or sneezing.

Promethazine was once used as an antipsychotic. However, this is not so common anymore.

Promethazine hydrochloride works by blocking histamine, the substance produced by your body during an allergic reaction. Yet, it also has an impact on other substances in the brain, such as acetylcholine. It’s thanks to these anticholinergic properties that promethazine has its calming, sedative effect. 

How Do You Take Promethazine?

Usually, promethazine is taken as a tablet, a capsule, or a liquid. These are typically available to buy at a pharmacy under brand names such as Avomine, Phenergan, Sominex, or more commonly Night Nurse (and Day & Night Nurse).

  • Taking promethazine for sleep problems: Commonly, Night Nurse capsules contain 20 milligrams (mg) of promethazine, which is often enough to help people get to sleep. However, you may require more, and other medicines contain up to 50 mg. 

If you are taking promethazine for sleep problems, you’ll need to take it about 20 minutes before going to bed. 

  • Promethazine for allergies: For allergic reactions and nausea, stronger dosages might be prescribed. If you are suffering from hay fever, you may be told to take promethazine twice or three times a day. Commonly, you’ll start with a 25 mg dose and then take regular 12.5 mg doses. 

Only take promethazine in the doses recommended on the medicine’s packaging. Do not double the amount you take to make up for a missed dose.

It is essential to know that promethazine is often mixed with other medicines, such as paracetamol, dextromethorphan, codeine, and pholcodine, to help treat coughs and pain. These can have their own side effects. 

Note: Promethazine can also be taken by intravenous injection and rectal suppository too. Ask your doctor for advice if they have prescribed this to you.

Promethazine Side Effects: What You Need to Know

Like all medicines, promethazine can have side effects. These are uncommon and tend to be more likely in children and the elderly. They will also be more likely if you increase your dose — or take more than is recommended.

The most common side effects of promethazine include:

  • Daytime sleepiness or drowsiness
  • Headache
  • Dizziness or unsteadiness (this is particularly common in children)
  • Nightmares or strange dreams
  • Difficulty concentrating
  • Restlessness or excitability. Strangely, for a sleep aid, promethazine can cause excitedness — particularly in children.

More severe side effects are less likely, but they can include:

  • Jaundice, or yellowing of the skin and eyes. It usually shows there is a problem with the liver.
  • Frequent bruising or bleeding
  • Muscle stiffness
  • Unusual movements of the face, mouth, and tongue
  • Anaphylaxis. This is a serious allergic reaction that can be life-threatening. You should immediately seek medical assistance if you have trouble breathing or your face swells.
  • Respiratory depression. It’s the technical term for hyperventilation, or slow, ineffective breathing. This is common in very young children. That’s why the US Food and Drugs Administration (FDA) does not recommend the drug to children under two years of age.

Meanwhile, older people are more likely to experience some of the following:

  • Dry mouth
  • Blurred vision
  • Confusion
  • Difficulty urinating

Finally, if you overdose on promethazine, you may experience some severe adverse effects, including:

  • Extreme drowsiness
  • Breathing problems
  • Fast heart rate or palpitations
  • Hallucinations

In these cases, it is crucial that you immediately seek medical advice.

Who Shouldn’t Take Promethazine?

While promethazine is a popular drug, it is not for everyone. If you are on certain medications or suffer from a particular medical condition, side effects might be more likely — and may be more severe.

Talk to a doctor before you take promethazine if any of the following apply to you:

  • You have an allergy to promethazine
  • You cannot have alcohol (some promethazine products contain very small amounts of alcohol)
  • You have glaucoma
  • You struggle to urinate
  • You are pregnant, trying to become pregnant, or you are breastfeeding
  • You are taking antidepressants, particularly monoamine oxidase inhibitors
  • You are on other medications that include paracetamol
  • You are taking medicines that make you drowsy, give you a dry mouth, or make it difficult to urinate.
  • You are on herbal remedies to help you sleep, or other sleep supplements.
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Key Takeaways

Promethazine is an effective drug for treating allergy symptoms, short-term sleep problems, nausea, and cold symptoms. While it is generally very safe, common promethazine side effects can include daytime drowsiness, nightmares, and headaches.

You must be aware of any possible drug interactions before you take promethazine. These can make side effects worse. 


Is Promethazine Safe to Take Daily?

Promethazine is safe to take daily, and it is often prescribed to be taken several times a day. However, many medications containing promethazine also contain things like paracetamol or codeine. You shouldn’t take these drugs more than is necessary — so make sure you only use the medicine when you need it. 

How Long Does It Take for Promethazine to Work?

If you are taking promethazine for sleep, you’re recommended to take the medicine at least twenty minutes before you go to bed. It generally takes little more than twenty minutes to make you feel sleepy. 

Can I Take Promethazine with Ibuprofen? 

It’s perfectly safe to take promethazine with ibuprofen, as well as with other painkillers such as paracetamol. Remember to read the label carefully and only take the recommended dose.

While we've ensured that everything you read on the Health Centre is medically reviewed and approved, information presented here is not intended to be a substitute for professional medical advice, diagnosis, or treatment. It should never be relied upon for specific medical advice. If you have any questions or concerns, please talk to your doctor.

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