Promethazine: what it does and when people use it

Promethazine (brand name Phenergan) is an older antihistamine that still gets used a lot. It helps with allergy symptoms, motion sickness, nausea and vomiting, and it’s sometimes used as a short-term sedative or to calm people before procedures. It works by blocking H1 histamine receptors and has anticholinergic and mild sedative effects.

Common uses and how it’s given

Promethazine comes as tablets, syrup, suppositories, and injectable forms. For allergies, people often take it by mouth once or twice a day. For nausea or motion sickness, smaller doses can be given before travel or symptoms start. In the clinic it can be given by intramuscular or intravenous injection—only by professionals. Talk to your doctor about the right form for your situation.

Typical adult oral doses are usually in the 12.5–25 mg range every 4–6 hours as needed. Children’s dosing depends on age and weight—do not give promethazine to kids under 2 years old because of serious breathing risks. Follow the exact dose your prescriber gives you; do not guess or split adult pills for children.

Side effects, risks, and interactions

Common side effects include drowsiness, dizziness, dry mouth, blurred vision, and constipation. Because it sedates, avoid driving or operating machines until you know how it affects you. Serious risks include low blood pressure, severe breathing problems (especially in young children and older adults), and extrapyramidal symptoms like muscle stiffness.

Promethazine interacts with alcohol and other central nervous system depressants—combining them can dangerously increase drowsiness or slow breathing. It can also interact with MAO inhibitors and certain antipsychotics. If you take opioids, benzodiazepines, or sleep aids, tell your prescriber before starting promethazine.

Pregnancy and breastfeeding need special caution. It’s usually avoided late in pregnancy because it can affect newborns. If you’re pregnant, breastfeeding, or planning pregnancy, ask your doctor if promethazine is right for you.

Allergic reactions to any medicine are possible. Stop the drug and seek immediate help if you get hives, swelling, trouble breathing, or high fever with stiff muscles.

Practical tips: take tablets with a little food if stomach upset occurs. Use the lowest effective dose for the shortest time. Store at room temperature away from moisture and keep out of reach of children. Properly dispose of unused medication—don’t flush unless the label says to.

If you’re unsure whether promethazine is safe for you, or if side effects are severe or worrying, call your healthcare provider or pharmacist. They can check interactions with other meds and suggest safer alternatives if needed.