Promethazine: uses, dosing, side effects & brands

Phenergan (Promethazine Hydrochloride) is a phenothiazine used to treat Anaphylaxis, Allergic Conjunctivitis, Motion Sickness, Nausea.

The FDA classifies promethazine as a phenothiazine. If you are checking whether it is safe to combine with something else, the class is what matters — two drugs from the same class usually should not be stacked.

The label states there are no adequate and well-controlled studies in pregnant women, and that promethazine should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus. Animal (rat) feeding studies did not demonstrate teratogenic effects at the doses tested. The label adds a nonteratogenic caution: given to a pregnant woman within two weeks of delivery, it may inhibit platelet aggregation in the newborn. Anyone who is or may become pregnant should discuss use with their prescriber. This is what promethazine's FDA label says; whether to use it in pregnancy is your prescriber's decision.

The elimination half-life of promethazine is about 9 to 16 hours — that is how long the body takes to clear half of a dose. Kinetics are linear (no saturable/dose-dependent behavior). The FDA label reports a plasma half-life of 9-16 hours after IV dosing and a mean of 9.8 ± 3.4 hours after IM dosing; the oral tablet label omits a numeric half-life but describes the same disposition. Duration of clinical effect (about 4-6 hours, occasionally up to 12) is shorter than the elimination half-life, so sedation can wear off well before the drug clears. Promethazine is metabolized in the liver to sulfoxides of promethazine and N-desmethylpromethazine (the main urinary metabolites); the label does not characterize these as clinically active or as outlasting the parent. Not a prodrug. Because clearance is hepatic, impaired liver function may prolong the half-life; the label gives no specific age- or kidney-based half-life values. Half-life is not the same as how long a drug test can detect the drug, and it varies with age, kidney and liver function.

It depends on the medicine. We check it against the FDA labels rather than guessing: our interaction checker searches each drug's own label for the other and quotes what it says, naming the section it came from. Run promethazine against whatever else you take — and remember that a label not naming a drug is not the same as that combination being safe.

We track 4 promethazine-containing products in the U.S.: Promethegan, Phenergan, Promethazine Hydrochloride and Promethazine Plain. They are the same active ingredient; they differ in form, manufacturer, price and FDA recall record.

Across the brands we track, promethazine is currently marketed as suppository, injectable, solution, syrup and tablet, per the FDA's National Drug Code Directory. Each form is dosed differently — follow the label for the exact product you were prescribed.

Yes. Our catalog lists 3 generic promethazine products alongside the brand versions. A generic has the same active ingredient and must meet the FDA's bioequivalence standard; it usually costs less. Ask your pharmacist which one your plan covers.

Drowsiness. The FDA label calls it "the most prominent CNS effect of this drug." Alongside it the label lists sedation, somnolence, blurred vision, dizziness, confusion, and disorientation. Anticholinergic effects are the other everyday group: dry mouth, and in overdose the label describes an atropine-like picture of dry mouth, fixed dilated pupils, and flushing. Also listed: nausea, vomiting, increased or decreased blood pressure, fast or slow heart rate, faintness, dermatitis, photosensitivity, and hives — the label tells patients to avoid prolonged sun exposure. Less commonly, promethazine does the opposite of sedating: hyperexcitability and abnormal movements have been reported after a single dose.

No. Promethazine is contraindicated under age 2 — not "use carefully," but do not use. This is one of the drug's two boxed warnings, and it appears on the oral tablet label in capital letters as well as on the injection label. The reason is fatal respiratory depression: the FDA label reports postmarketing cases of respiratory depression, including deaths, in children under 2. There is no safe smaller dose to fall back on, because the label states that a wide range of weight-based doses produced respiratory depression in these children. For children 2 and older it is still not routine — use the lowest effective dose, and avoid giving it with any other drug that depresses breathing, which the label links to respiratory depression and sometimes death in children.

Because the injectable form can destroy tissue at and around the injection site. The second boxed warning states that severe chemical irritation and tissue damage has occurred by every parenteral route, including gangrene, tissue necrosis, and thrombophlebitis, and that some patients have required fasciotomy, skin grafting, or amputation. Three routes are flatly contraindicated: intravenous at concentrations above 1 mg/mL, intra-arterial, and subcutaneous. The preferred route is deep intramuscular. This risk belongs to the shot, not to tablets or syrup. If you are receiving a promethazine injection or infusion, pain or burning at the site is a warning sign, not something to sit through — tell staff immediately, because the label directs them to stop the infusion at once and check for arterial injection or leakage into surrounding tissue.

No to driving until you know how it affects you, and no to alcohol. The label warns that promethazine may cause marked drowsiness and may impair the mental and physical abilities needed for hazardous tasks such as driving a vehicle or operating machinery. It also states the impairment may be amplified by alcohol and other CNS depressants — sedatives and hypnotics including barbiturates, general anesthetics, narcotics and narcotic analgesics, tricyclic antidepressants, and tranquilizers — and that such agents should be eliminated or given at reduced dosage. Two more things worth knowing: older adults are more prone to confusion and over-sedation and should start on low doses, and naloxone does not reverse promethazine's depressant effects.

Per the FDA label, promethazine is contraindicated in: children under 2 years of age; anyone in a comatose state; and anyone with known hypersensitivity or a past idiosyncratic reaction to promethazine or other phenothiazines. The oral label adds that antihistamines are contraindicated for treating lower respiratory tract symptoms including asthma. The injection additionally must not be given intra-arterially, subcutaneously, or intravenously above 1 mg/mL. Beyond outright contraindications, the label says to avoid promethazine in people with compromised respiratory function or at risk of respiratory failure, such as COPD or sleep apnea, because of potentially fatal respiratory depression — and, because the drug is anticholinergic, to use caution with narrow-angle glaucoma, an enlarged prostate, bladder-neck obstruction, stenosing peptic ulcer, or pyloroduodenal obstruction. Get emergency help for high fever with muscle rigidity and altered mental status, which the label describes as neuroleptic malignant syndrome and calls potentially fatal.

Promethazine (brand Phenergan) is an antihistamine that is FDA-approved for several uses: relieving allergy symptoms, preventing and treating nausea and vomiting, preventing motion sickness, sedation before or after surgery, and as an add-on to pain control after surgery. It causes drowsiness, which is part of how it calms nausea and allergic reactions.

Yes — treating nausea and vomiting is an approved use, and it is also combined with other ingredients in some prescription cough-and-cold products. Because it makes most people very drowsy, it is often taken at bedtime. It should not be given to children under 2 years old due to the risk of serious, sometimes fatal, breathing problems.

Use extreme caution. Promethazine adds to the sedating and breathing-slowing effects of opioids, alcohol, and other sedatives, which can be dangerous. Promethazine-codeine cough syrup in particular is often misused. Always tell your doctor about every sedating medication or alcohol use before taking it.