Promethazine (Phenergan) can cause side effects ranging from common and expected to rare and serious. If you are looking for a promethazine safety guide, this page gives a clear UK-focused overview of what is normal, what to monitor, and when to get urgent medical help, whether you are taking promethazine yourself, considering it, or helping an older relative who uses it. It organises promethazine side effects by severity and body system, and also points out important risks such as drowsiness and other anticholinergic effects, drug interactions, extra caution in elderly patients, and simple steps for managing mild side effects safely.
One thing worth knowing upfront: UK product information only publishes a specific frequency for one side effect, sedation, which is listed as "very common". Everything else is listed as "frequency not known", meaning no reliable percentage exists for how often it happens, so treat any specific percentage figure you see elsewhere with caution. Knowing that matters because it helps you judge side effects more safely, avoid risky combinations with other medicines, and act quickly if symptoms suggest something more serious.
Key things to know
- Sedation is the only side effect with a confirmed frequency in UK product information, listed as "very common".
- Anticholinergic effects such as dry mouth, blurred vision and constipation are well documented, although some can be more serious.
- Elderly patients have increased sensitivity to sedative and anticholinergic effects, with greater risks of confusion and falls.
- It must not be used in children younger than 2 years of age because it can cause severe respiratory depression.
- Restlessness and involuntary movement (akathisia) are specifically documented for promethazine.
- It must not be taken within 14 days of an MAOI medicine, and alcohol may increase drowsiness when combined with promethazine, alongside the added risk of other sedating medicines
Common Side Effects
Drowsiness is the most consistently reported effect, listed as "very common" in UK product information. Other reported effects include dizziness and dry mouth. Drowsiness can persist into the next day, and the sedating effect can last up to 12 hours, so if you feel drowsy, avoid driving or operating machinery.
Headaches and difficulty concentrating have also been reported.
Anticholinergic Effects
Promethazine has anticholinergic activity, meaning it blocks certain receptors involved in various bodily functions. This can cause dry mouth, blurred vision and constipation. Less commonly reported effects include urinary retention and, in some cases, paralytic ileus, which requires medical attention.
Cardiovascular Effects
Promethazine can affect heart rhythm, including QT interval prolongation, particularly in susceptible individuals or when combined with other medicines that affect heart rhythm. Blood pressure can also drop on standing (orthostatic hypotension), causing dizziness, which is particularly relevant for elderly patients at higher falls risk.
Neurological and Psychiatric Effects
Beyond sedation, some people experience confusion, particularly older adults. Paradoxical reactions such as agitation, restlessness, or vivid dreams have also been reported, particularly in children and older adults.
Restlessness and involuntary movements (akathisia) have been reported with promethazine. This is different from clinical restless legs syndrome. If you develop a new or troubling sense of inner restlessness or an urge to keep moving, mention it to your prescriber.
Promethazine has also been linked to neuroleptic malignant syndrome, a serious condition requiring urgent medical attention if suspected.
Extra Risks for Elderly Patients
Older adults are more sensitive to promethazine's anticholinergic and sedative effects, and with increased risks of falls, confusion and orthostatic hypotension. Older people should discuss its use with a healthcare professional, particularly if they have other medical conditions or take other medicines. If promethazine is genuinely needed in an older adult, a lower dose and closer monitoring, decided by a prescriber, is the appropriate approach rather than avoiding the conversation or self-managing the dose.
Drug Interactions Worth Knowing About
Combining promethazine with other central nervous system depressants, including benzodiazepines or opioids, can increase sedation and may increase the risk of respiratory depression.
It must not be taken within 14 days of an MAOI (monoamine oxidase inhibitor) medicine, which can cause dangerous blood pressure changes. Promethazine can also interact with other medications. Other anticholinergic medicines can compound dry mouth, constipation, and confusion. Before taking promethazine or combining it with other drugs, tell your healthcare provider about any other medications, over-the-counter products, vitamins, herbal products, and supplements you use.
When to Seek Urgent Medical Help
Most side effects are mild and manageable, but seek urgent medical attention for serious side effects if you develop the following symptoms: a serious allergic reaction (swelling of the face, lips or throat, widespread rash), severe or worsening confusion, trouble breathing, shallow breathing, or unusually slow breathing, or involuntary muscle movements that are severe or distressing.
Breathing problems are particularly important in children and in people with respiratory conditions, because promethazine can affect respiratory function.
Promethazine should not be given with codeine to children under 16, and you should get urgent medical advice if this combination has been used. Rare but severe allergic reactions, including anaphylaxis, need immediate help.
Managing Milder Side Effects
If you experience persistent or bothersome side effects, speak to your pharmacist or prescriber, who can advise whether a change in dose, formulation or treatment is appropriate.
Medical Disclaimer
This page provides general information about promethazine side effects. It is not a substitute for personalised medical advice. Seek urgent medical attention for signs of a severe allergic reaction, difficulty breathing, or severe confusion. Do not adjust your dose or stop treatment without speaking to your prescriber or pharmacist, unless you are experiencing a serious reaction requiring urgent medical attention.
