Phenergan contains promethazine hydrochloride, and the right dose depends on why you are taking it. The UK Phenergan 25mg tablet information does not give one overall maximum dose that applies to every adult and every use. Instead, the recommended dose changes for allergies, motion sickness and short term insomnia.
For example, adults taking Phenergan 25mg tablets for allergies usually start with 25mg at night. This can be increased to 25mg twice a day if needed. For short term insomnia, the recommended dose is 25mg or 50mg as a single night time dose. For motion sickness, 25mg is usually taken the night before a journey and can be repeated after 6 to 8 hours if needed.
This guide covers the usual adult doses, why the dose changes depending on the condition, what to do if you take more than recommended, how long the effects can last, important interactions, children's use, pricing and when to get medical advice.
Key things to know
- Adult doses depend on why you are taking Phenergan
- For allergies, adults taking Phenergan 25mg tablets usually start with 25mg at night, with a maximum of 25mg twice a day for this use
- For short term insomnia, the usual adult dose is 25mg or 50mg as a single night time dose
- For motion sickness, 25mg is usually taken the night before travel and can be repeated after 6 to 8 hours if needed
- There is no single 100mg daily maximum for all oral Phenergan uses
- Phenergan can cause significant drowsiness, so take care with driving and other activities that require alertness
- Phenergan should not be used for more than 7 days without medical advice
- The Phenergan 25mg tablets covered here are not for children under 6
Standard Adult Dosing
| Use | Typical adult dose |
|---|---|
| Allergic conditions such as hay fever and hives | 25mg at night, increasing to a maximum of 25mg twice a day if needed |
| Short term sleep difficulties | 25mg or 50mg as a single night time dose, for no more than 7 days without medical advice |
| Motion sickness prevention | 25mg the night before a journey, repeated after 6 to 8 hours if needed |
| Sedation before surgery | Dosing is decided by the clinical team and depends on the type of procedure and patient |
These are the doses given in the UK Phenergan 25mg tablet information for the uses covered above. Your doctor or pharmacist may give you different instructions depending on your circumstances and the particular promethazine product you are using.
The dose for one condition is not automatically suitable for another. For example, the dose used for hay fever is different from the dose used for short term insomnia. Follow the instructions on your medicine or the advice given by your doctor or pharmacist rather than using a dose from another section of this guide.
Is There a Maximum Phenergan Dose in 24 Hours?
There is not one overall 100mg maximum that applies to every Phenergan tablet or liquid dose.
For Phenergan 25mg tablets, the maximum depends on the reason for treatment. For allergies, the adult dose can be increased to 25mg twice a day. For motion sickness, 25mg can be repeated after 6 to 8 hours if required. For short term insomnia, 25mg or 50mg is taken as a single night time dose.
You may see 100mg described as a maximum dose for Phenergan elsewhere. The UK prescribing information for Phenergan injection lists 100mg as the maximum parenteral dose. That does not mean that adults taking Phenergan tablets or elixir should take 100mg in 24 hours.
This distinction matters because different formulations and different medical uses can have different dosing instructions.
Is 100mg of Phenergan Too Much?
100mg is well above the usual oral doses listed for Phenergan tablets for allergies, motion sickness or short term insomnia. It should not be treated as a routine adult dose or as a general daily target.
If you have taken more Phenergan than recommended, contact 111 for advice even if you feel well. NHS guidance recommends getting advice after taking more than the recommended dose, even when there are no symptoms.
Call 999 or go to A&E if you or someone else has breathing problems, a seizure, becomes unconscious, has a very fast or irregular heartbeat, or develops severe uncontrolled movements or hallucinations after taking too much.
How Long Does the Effect Last?
Promethazine is a long acting antihistamine and can cause drowsiness. The effects can last for several hours, and the Phenergan prescribing information notes that sedation or sleepiness is a very common side effect.
This matters if you need to drive, work, study or do anything that requires you to stay alert. Do not drive or operate machinery if Phenergan makes you sleepy, dizzy or otherwise affects your alertness.
What About Children?
The Phenergan 25mg tablets covered on this page are not for children under 6. The product information gives different doses for children aged 6 to 10 and for those over 10, depending on why the medicine is being used.
Other promethazine products have different age restrictions. For example, NHS guidance says some promethazine liquid products may be prescribed for children aged 2 to 5, while some products have higher minimum ages. This is why you should check the exact product rather than using a general children's dose found online.
If you are giving promethazine to a child, ask a doctor or pharmacist to confirm the correct product and dose for their age and the reason it is being used.
Drug Interactions and Risk of Excessive Sedation
Promethazine can make you sleepy, and combining it with alcohol or other medicines that cause drowsiness can increase this effect. The Phenergan prescribing information also says to avoid MAOIs during treatment and for 14 days after taking them.
Tell your doctor or pharmacist about any medicines you take regularly, particularly sleeping tablets, sedatives, some antidepressants, strong painkillers or other medicines that can make you drowsy.
Phenergan can also interact with medicines that affect the heart rhythm and with medicines that have anticholinergic effects. Your pharmacist can check your medicines before you start treatment.
If you are unsure about your dose or whether another medicine can be taken with Phenergan, ask a pharmacist before taking the next dose.
Genuine Pricing
| Product | Price |
|---|---|
| Phenergan 25mg, 56 tablets | £14.99 |
| Phenergan Elixir 5mg/5ml, 100ml | £14.49 |
Medical Disclaimer
This page provides general information about adult promethazine dosing. It is not a substitute for personalised medical advice, diagnosis or treatment.
Do not change your dose yourself if you feel that Phenergan is too weak or too strong. The medicine leaflet specifically advises speaking to a doctor or pharmacist instead. Do not use Phenergan for longer than 7 days without medical advice.
Possible side effects include drowsiness, dizziness, dry mouth, blurred vision and constipation. Less common reactions can include movement problems such as dystonia. Serious reactions can occur after an overdose, including seizures, changes in heart rhythm, loss of consciousness and breathing problems.
If you have taken more than the recommended dose, contact 111 even if you feel well. If you have breathing problems, a seizure, loss of consciousness or a very fast or irregular heartbeat, call 999 or go to A&E.
