Promethazine: what it treats, how to take it, side effects
Last updated September 3, 2026.
Promethazine is a phenothiazine antihistamine - it blocks the action of a natural substance behind allergy, nausea, and motion sickness. You may know it as Phenergan. It is one of the most versatile old drugs in the cabinet, and it carries one of the most serious child warnings in the formulary.
What does it treat?
Allergic reactions - runny nose, watery eyes, allergic skin reactions - plus anaphylaxis alongside other medications, cold symptoms, nausea and vomiting after surgery, sedation before and after surgery and during labor, and motion sickness. It controls symptoms; it does not treat the cause or speed recovery.
How do you take it?
It depends on the job: allergies run one to four times daily before meals and at bedtime; cold symptoms and nausea run every 4 to 6 hours as needed; motion sickness starts 30 to 60 minutes before travel, repeats after 8 to 12 hours if needed. It comes as tablets, syrup (measure with the dosing device, never a kitchen spoon), and rectal suppositories - suppositories are rectal use only. Missed dose: take it when you remember unless the next one is close; never double up.
Side effects to know
- Common: drowsiness, dry mouth, dizziness, listlessness, nightmares, blurred or double vision, loss of coordination, restlessness, and a stuffy nose.
- Serious - call your doctor immediately: wheezing, slowed breathing, or breathing that stops for short periods; fever with sweating, stiff muscles, and decreased alertness; fast or irregular heartbeat, faintness; uncontrollable movements, uncontrolled eye movements, tongue sticking out, or an abnormal neck position; hallucinations or confusion; seizures; unusual bruising or bleeding; sore throat with fever and chills; and yellowing of the skin or eyes.
Who should not take it?
The warning that leads the page: promethazine can slow or stop breathing and can cause death in children - never in children under 2, with caution at 2 and older, and never routinely for vomiting in children. Any breathing condition in a child - lung disease, asthma, sleep apnea - goes to the doctor first. Adults 65 and older are steered to safer alternatives. Flag enlarged prostate, glaucoma, seizures, ulcers, stomach or bladder blockage, asthma or lung disease, sleep apnea, bone-marrow conditions, and heart or liver disease. Sedatives, sleeping pills, narcotic pain medications, and anxiety medications all stack on its breathing effects - the full list goes to your doctor and pharmacist. Alcohol makes its side effects worse: ask your doctor about it directly. It causes real drowsiness, so no driving, and it sensitizes skin to sunlight. If you become pregnant, call your doctor.
When is it an emergency?
Slowed breathing, pauses in breathing, or blue lips is a 911 call - in a child or an adult. A seizure, unresponsiveness, or the stiff-muscles-with-fever picture is the same. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Why is promethazine considered dangerous for children?
The MedlinePlus warning is direct: promethazine may cause breathing to slow or stop, and may cause death in children. It is never given under age 2, given with caution at 2 and older, and combination products with codeine are barred under 16. It is also not for routine childhood vomiting - only specific cases a doctor chooses. If a child on it has difficulty breathing, wheezing, or pauses in breathing, that is emergency care immediately.
Why do I have to take promethazine before travel instead of when I feel sick?
It prevents better than it rescues. MedlinePlus times motion-sickness dosing 30 to 60 minutes before travel, with a repeat after 8 to 12 hours if needed. The drug blocks the signal before the cascade starts; once vomiting begins, the same pill works slower and weaker. On long trips it runs morning and before the evening meal, each travel day.
Can I drink alcohol while taking promethazine?
Ask your doctor first - MedlinePlus says alcohol can make the side effects of promethazine worse, and the side effect in question is sedation stacked on sedation, plus worse breathing suppression. This is a drug that already tells you not to drive. The honest default is none while taking it.
Why did my doctor mention my other medications three separate times?
Because promethazine slows breathing and sedates, anything else that does the same - narcotic pain relievers, anxiety medications, sedatives, sleeping pills, tranquilizers, barbiturates - multiplies the effect rather than adding it. MedlinePlus says some medications should not be taken with it at all and others need dose changes or monitoring. The full list, vitamins and herbals included, is the price of a safe prescription.
Why am I suddenly burning in the sun on promethazine?
It sensitizes skin to sunlight. MedlinePlus advises avoiding unnecessary or prolonged sun and wearing protective clothing, sunglasses, and sunscreen while taking it. A short errand can become a burn faster than you expect. The sensitivity lasts as long as you are on the drug.
What happens if I miss a dose of promethazine?
Take it as soon as you remember, unless it is almost time for the next one - then skip it and never double up. Most of its uses are as-needed anyway, so the schedule follows the symptom: allergies and colds on their routines, motion sickness on the travel clock.