Phenergan side effects — what to expect and how to stay safe

Phenergan (promethazine) is a strong antihistamine used for nausea, motion sickness, allergies and to help with sleep before procedures. It works fast, but it also causes side effects that catch people off guard. Want the short version? Expect drowsiness and be careful around alcohol, driving, and children.

Common side effects you’ll notice

Drowsiness is the most common effect — many people feel sleepy within 30–60 minutes and stay groggy for several hours. You may also get dizziness, dry mouth, blurred vision, constipation, or urinary trouble. Some people feel lightheaded when standing up (low blood pressure) or notice increased sensitivity to the sun.

These are usually manageable: avoid driving or operating heavy machinery, sip water for dry mouth, and rise slowly from sitting to reduce dizziness. If side effects are mild but annoying, ask your prescriber about a lower dose or an alternative medicine.

Serious risks and when to call for help

Phenergan can cause dangerous problems in some people. Babies and very young children are at risk for severe breathing problems — promethazine should not be used in children under 2, and doctors warn caution up the age range. Older adults can become very confused, unsteady, or suffer falls because of strong sedation and low blood pressure.

Watch for signs that need urgent care: trouble breathing, very slow breathing, fainting, severe dizziness, a fast or irregular heartbeat, high fever with stiff muscles or extreme confusion (these can be rare drug reactions). If you get a rash, swelling of the face or throat, or any sign of an allergic reaction, seek emergency help right away.

Promethazine can also cause movement problems (like tremor or muscle stiffness) and, rarely, heart rhythm issues if mixed with other drugs that affect the QT interval. Tell your doctor about any heart conditions, seizures, or medications you already take.

Drug interactions matter. Don’t mix Phenergan with alcohol, opioids, benzodiazepines, or other sedatives — combining these can dangerously slow breathing and boost sedation. Some antidepressants and certain antibiotics can interact and raise the risk of heart rhythm changes. Always list every medicine, supplement, and herb to your prescriber.

If you’re pregnant or breastfeeding, ask your health provider before taking Phenergan. It may be used in pregnancy for severe nausea, but the risks and benefits need a clear discussion. For breastfeeding, promethazine can pass into milk and make the baby sleepy.

Want to avoid side effects? Use the lowest effective dose, take it only when needed, skip alcohol, and avoid driving until you know how it affects you. If side effects are severe or do not settle, call your doctor — they can recommend an alternative anti-nausea drug or adjust dosing.

If you have questions about Phenergan, bring a full list of your medicines to your pharmacist or doctor. They can check for interactions and suggest safer options based on your age, health history, and other drugs you take.