Promethazine: What It Does and How to Use It Safely

Promethazine is a first-generation antihistamine that many people use for allergies, nausea, motion sickness, and as a mild sedative. It’s been around for decades and shows up in pills, syrups, and injections. If you’ve heard it called an antiemetic or a sleep aid, that’s promethazine doing its job.

How it works is simple: promethazine blocks histamine receptors and calms parts of the brain that trigger vomiting and wakefulness. That makes it useful after surgery, for chemo-related nausea, or during a rough flight. It can also help with itchy rashes and allergic reactions that don’t need emergency care.

Common doses and forms

Promethazine comes as tablets (often 10–25 mg), oral syrup, rectal suppositories, and injections. Typical adult doses vary by use: for allergies 10–25 mg every 4–6 hours as needed, and for nausea 12.5–25 mg every 4–6 hours. Children’s doses are smaller and must be calculated by weight. Never give injectable promethazine at home unless directed by a healthcare professional.

Side effects and safety warnings

Drowsiness is the most common side effect — promethazine can make you very sleepy. Other effects include dry mouth, blurred vision, dizziness, and constipation. It can lower blood pressure in some people and cause breathing problems in young children. Don’t drive or operate heavy machinery until you know how it affects you.

There are important safety points: avoid promethazine in children under 2 years because of the risk of severe breathing problems. Older adults may be more sensitive to confusion, low blood pressure, and falls. If you’re pregnant, breastfeeding, or have severe liver disease, talk to your doctor before using it. Combining promethazine with alcohol, opioids, benzodiazepines, or other sedatives greatly increases the risk of extreme drowsiness and breathing trouble.

Also watch for allergic reactions like swelling of the face, trouble breathing, or a fast heartbeat. If any of those happen, get emergency help right away.

Overdose signs include extreme drowsiness, agitation, hallucinations, slow breathing, or a dangerously low heart rate. Call emergency services or your poison control center if you suspect an overdose.

Buying promethazine online: only use licensed pharmacies and avoid sites that don’t require a prescription when one is needed. Fake or poor-quality meds can be dangerous. If ordering by mail, check reviews, pharmacy registration, and return policies. When in doubt, speak with a pharmacist.

Tips to use promethazine smartly: start with the lowest effective dose, avoid mixing with other sedatives, and tell your doctor about all your medicines and conditions. Keep a list of side effects and check in with your prescriber if nausea or sleep problems don’t improve.

Want more on related meds or safe online pharmacies? Our site has guides on antihistamines, alternatives, and tips for ordering medicines safely. If you have specific health concerns, chat with a healthcare professional — they can tailor advice to your situation.