Phenergan Side Effects: What to Expect and When to Call Your Doctor

Promethazine (Phenergan) is an effective antihistamine and antiemetic, but it comes with a notable side effect profile that patients should understand before starting treatment. The most common side effect — drowsiness — is well-known, but serious and even life-threatening reactions are possible in some situations. This guide covers what to expect, what to watch for, and when to seek emergency help.

Important Black Box Warning

The FDA has issued a Black Box Warning — the strongest type of drug warning — for promethazine:

Children under 2 years: Promethazine is contraindicated in children under 2 years of age due to the risk of fatal respiratory depression. It should be used with extreme caution in children 2 years and older.

IV administration: Intravenous (IV) injection of promethazine is not recommended due to risk of severe tissue injury, gangrene, and potential amputation from inadvertent intra-arterial injection or extravasation.

Common Side Effects of Phenergan

These side effects are common and typically improve as your body adjusts to the medication. Let your provider know if they persist or worsen:

Drowsiness and sedation — The most frequent side effect. Do not drive or operate machinery until you know how promethazine affects you.

Dizziness — Stand up slowly from sitting or lying positions to reduce dizziness, especially in elderly patients.

Confusion or disorientation — Particularly common in older adults, who are more sensitive to promethazine's anticholinergic effects.

Dry mouth — An anticholinergic effect. Staying hydrated, using sugar-free gum or candy, and drinking water frequently can help.

Blurred vision — Also anticholinergic in nature. Avoid activities requiring clear vision until this resolves.

Constipation — Common with anticholinergic medications. Increased fiber intake and hydration can help.

Nasal congestion — Stuffy or runny nose may occur in some patients.

Serious Side Effects — Call Your Doctor Promptly

These side effects require prompt medical attention. Contact your provider if you experience:

Extrapyramidal symptoms (EPS) — Promethazine can cause involuntary muscle movements such as tremors, muscle stiffness, restlessness (akathisia), or abnormal posturing (dystonia). These require evaluation and may be treated with diphenhydramine or benztropine.

Tardive dyskinesia — With long-term use, repetitive involuntary movements (grimacing, tongue movements, lip smacking) may develop.

Jaundice or dark urine — May indicate liver problems. Stop taking promethazine and contact your provider.

Irregular heartbeat or palpitations — Promethazine may alter cardiac conduction. Report any heart rhythm irregularities to your doctor.

Unusual bruising or fever — Can signal blood cell problems (leukopenia) or infection. Rare but requires evaluation.

Life-Threatening Side Effects — Seek Emergency Care Immediately

Call 911 or go to the nearest emergency room if you experience:

Severe breathing difficulty — Promethazine can cause slow or shallow breathing, particularly at high doses or when combined with other CNS depressants like opioids or benzodiazepines.

Bluish discoloration of lips or fingernails (cyanosis) — This is a sign of dangerously low blood oxygen. Emergency care needed immediately.

Neuroleptic Malignant Syndrome (NMS) — A rare but potentially fatal reaction characterized by high fever, severe muscle stiffness, sweating, rapid heartbeat, and confusion. Stop promethazine immediately and seek emergency care.

Seizures — Promethazine can lower the seizure threshold. If you have a history of seizure disorders, discuss this risk with your provider before starting treatment.

Severe allergic reaction (anaphylaxis) — Signs include hives, difficulty breathing, swelling of face/lips/tongue/throat. Emergency care needed.

Special Populations: Side Effect Risks to Know

Elderly patients: Higher risk of falls from sedation and dizziness; greater sensitivity to anticholinergic effects (confusion, urinary retention). Promethazine is on the Beers Criteria as potentially inappropriate for older adults.

Children (2+ years): Respiratory depression risk is elevated. Use the lowest effective dose; do not give to children under 2.

Pregnancy: Safety in pregnancy is not fully established, but promethazine has been used for decades for pregnancy nausea. Discuss risks and benefits with your OB-GYN, especially in the first trimester.

Tips for Minimizing Side Effects

Take promethazine at bedtime when possible — the sedation effect can aid sleep rather than disrupt your day

Avoid alcohol and other sedatives, opioids, or CNS depressants when taking promethazine — additive sedation can be dangerous

Start with the lowest effective dose and adjust upward only as directed by your provider

Take with food or milk if stomach upset occurs

When Should I Call My Doctor?

Call your provider if: any side effect is severe, persistent, or worsening; you develop new symptoms after starting promethazine; or you're unsure whether something you're experiencing is related to the medication. When in doubt, always err on the side of contacting your healthcare team. For more information on how promethazine works and its uses, see What Is Phenergan? Uses, Dosage, and What You Need to Know