Promethazine. What diseases does it treat?

Promethazine is a phenothiazine antihistamine with antiemetic and sedative properties that blocks H1 receptors and has anticholinergic activity. Oral onset typically occurs within 20–60 minutes, peak effects appear around 1–3 hours, and clinical effect commonly lasts 4–6 hours. Clinicians choose promethazine when rapid symptomatic relief of histamine-mediated itching, urticaria, or allergic rhinitis is needed, or when a reliable antiemetic with sedative benefit is preferred.

For adults, common oral dosing ranges 12.5–25 mg every 4–6 hours as needed, with a usual maximum of 100 mg/day. For motion sickness, take 25 mg about 30–60 minutes before travel; repeat dosing only if required and within the daily limit. Parenteral administration (IM/IV) is reserved for severe nausea or when oral intake is not possible; follow product labeling and local protocols because injectable promethazine can cause severe local tissue injury and carries a boxed warning for young children.

Avoid promethazine in children under 2 years because of the risk of fatal respiratory depression. Use lower starting doses for older adults and patients with hepatic impairment due to increased sedation and anticholinergic burden. Do not combine promethazine with other strong CNS depressants, opioids, or alcohol without close supervision; the combination increases the risk of profound sedation and respiratory depression. Consult a prescriber before use in pregnancy or breastfeeding and use the minimum effective dose when a clinician approves therapy.

Expect common adverse effects such as drowsiness, dizziness, dry mouth, blurred vision, constipation, and urinary retention. Watch for less common but serious reactions: extrapyramidal symptoms, severe respiratory depression, and severe local tissue injury after parenteral administration. Review a patient’s medication list for interactions that lower seizure threshold or prolong sedation and adjust therapy accordingly.

Practical approach: start with the lowest effective dose, prefer oral administration for mild-to-moderate symptoms, give 25 mg pre-travel for motion sickness when appropriate, monitor respiration in high-risk patients, and seek urgent care for breathing difficulties or signs of tissue damage following injection. Discuss alternatives (second-generation antihistamines, ondansetron for nausea) if sedation or anticholinergic effects limit tolerability.

Promethazine for Motion Sickness: Recommended Doses and When to Take

Take 25 mg orally 30–60 minutes before travel; if symptoms recur during long trips, take 25 mg every 12 hours, not to exceed 100 mg in any 24‑hour period.

Children under 2 years must not receive promethazine. Typical pediatric dosing (use exact product concentrations and a measuring device): ages 2–5 years: 6.25 mg per dose; ages 6–11 years: 12.5–25 mg per dose; repeat per product label or provider instructions, observing maximum daily limits advised by the prescriber.

When rapid onset is needed or oral intake is impossible, give intramuscular injection of 12.5–25 mg; onset is faster than oral. Intravenous administration can cause local irritation and should follow institutional protocols; use IM instead unless IV is specifically indicated by a clinician.

Adjust doses for older adults: start at the lower end (12.5–25 mg) because of greater sensitivity to sedation and orthostatic hypotension. Reduce dose or avoid use in patients with severe lung disease, significant CNS depression, or prior paradoxical reactions to phenothiazines.

Avoid combining promethazine with alcohol, opioids, benzodiazepines, or other sedating drugs because of additive drowsiness and respiratory depression. Do not drive or operate heavy machinery until you know how the drug affects you.

Use during pregnancy or breastfeeding only under clinical guidance: promethazine crosses the placenta and appears in breast milk; prescribers commonly weigh benefits against potential neonatal sedation and respiratory effects.

If nausea persists despite proper dosing, contact a healthcare provider for alternative antiemetic strategies or further evaluation; stop promethazine and seek immediate care for signs of severe allergic reaction, respiratory difficulty, or extreme drowsiness.

Promethazine for Nausea and Vomiting in Chemotherapy: Timing, Dosing, and Rescue Strategies

Use promethazine as a rescue antiemetic for breakthrough chemotherapy-induced nausea and vomiting (CINV): give 12.5–25 mg PO/IV/IM at symptom onset and repeat every 4–6 hours as needed, with a maximum of 100 mg in 24 hours.

For timing, reserve promethazine primarily for breakthrough or refractory episodes during the acute (0–24 h) and delayed (24–120 h) phases rather than as the preferred prophylactic agent for highly emetogenic regimens. Give a single oral dose 30–60 minutes before chemotherapy only when patients lack access to guideline-directed prophylaxis and require short-term control.

Choose route by clinical context: PO for mild-moderate breakthrough nausea if the patient tolerates oral intake; IM for rapid effect when vomiting prevents oral dosing; IV for severe vomiting or when rapid onset is necessary, injecting slowly and monitoring respiratory status. Avoid intra-arterial or subcutaneous administration because of tissue injury risk.

Adjust dose for age and organ function: start at 12.5 mg in elderly patients and titrate cautiously; avoid use in children under 2 years; in hepatic impairment or severe frailty reduce dose and increase monitoring for sedation and hypotension.

Screen for interactions and safety risks before dosing: avoid or reduce concurrent CNS depressants (opioids, benzodiazepines) because additive respiratory depression and sedation occur; use caution with other QT-prolonging agents and anticholinergic drugs; do not combine with MAO inhibitors without specialist advice.

Follow a rescue sequence if promethazine provides incomplete control: 1) repeat one additional 12.5–25 mg dose after 4 hours if partial response; 2) if symptoms persist, add or switch to an alternative class such as an NK1 antagonist (aprepitant), olanzapine 5–10 mg (single dose or nightly for 2–3 days), or metoclopramide 10 mg IV/PO, considering extrapyramidal prophylaxis where appropriate; 3) for continuous vomiting, provide IV fluids, correct electrolytes, and escalate to inpatient intravenous antiemetic infusion or specialist review.

Monitor for adverse effects: check sedation level, respiratory rate and blood pressure after parenteral dosing; watch for anticholinergic effects (dry mouth, urinary retention), extrapyramidal symptoms, and signs of excessive QT interval prolongation. Stop promethazine and seek urgent review for severe sedation, respiratory compromise, or neuroleptic malignant syndrome–like features.

Document response and plan: record time and dose of each promethazine administration, symptom relief and side effects, then revise the antiemetic regimen for subsequent cycles–prefer guideline-based prophylaxis for future treatments rather than routine reliance on promethazine alone.

Allergic Symptoms Addressed by Promethazine: Which Signs Improve and When to Seek Emergency Care

Use promethazine to relieve itching, hives (urticaria), sneezing, nasal discharge and allergic eye tearing; do not rely on it for airway or circulatory collapse–treat suspected anaphylaxis with intramuscular epinephrine and call emergency services immediately.

What improves and how fast:

• Itch and skin discomfort: oral relief often begins within 15–60 minutes, with noticeable reduction in pruritus by 30–90 minutes.
• Hives (wheal and flare): itching and redness usually lessen within 1–3 hours; reduction in new lesion formation occurs over several hours, but complete clearing may take longer.
• Sneezing, runny nose and watery eyes: symptom reduction typically starts within 30–90 minutes after oral dosing.
• Allergic conjunctivitis symptoms improve in parallel with nasal symptoms; topical ophthalmic therapy or a clinician-prescribed regimen may still be needed.
• Onset by route: oral 15–60 minutes (peak 2–4 hours, duration ~4–6 hours); intramuscular 15–30 minutes; intravenous administration produces rapid effect but carries higher risk and should be reserved for controlled settings.

Recommended adult dosing guidance for allergic symptoms (general reference only):

• Typical oral dose: 25 mg every 4–6 hours as needed.
• Maximum usual daily dose: 100 mg for most adults; do not exceed this without clinician direction.
• Avoid use in children younger than 2 years; follow pediatrician dosing for older children and infants by weight if indicated.

When promethazine is insufficient or inappropriate:

• Large, rapidly spreading hives, angioedema of the face or tongue, wheeze or noisy breathing require immediate emergency care–use epinephrine first if anaphylaxis is suspected.
• Severe throat tightness, difficulty swallowing, lightheadedness or fainting indicate compromised airway or circulation; call emergency services without delay.
• Marked sedation, slowed breathing, confusion or loss of consciousness after taking promethazine merits urgent evaluation, especially when combined with opioids, benzodiazepines or alcohol.
• New rash with fever, blistering, or mucosal involvement may signal a serious drug reaction and needs urgent medical assessment.

Practical stepwise actions for allergic episodes:

1. For mild-to-moderate itching or hives: take promethazine per dose guidance, apply cool compresses, and avoid known triggers.
2. If symptoms worsen (shortness of breath, facial/tongue swelling, syncope) administer epinephrine immediately if available and call emergency services.
3. If promethazine causes excessive drowsiness, breathing difficulty or agitation, stop the drug and seek urgent care.
4. If hives persist or recur daily or last beyond 48–72 hours despite antihistamine treatment, contact a clinician for further evaluation and consider second-line options (alternative antihistamines, prescription regimens, or corticosteroids as indicated).

Important safety notes and interactions:

• Do not use promethazine intravenously outside a hospital setting due to risk of severe tissue injury and respiratory depression.
• Avoid combining promethazine with alcohol, opioids or sedative-hypnotics because of additive respiratory and cognitive depression.
• Older adults face higher risk of confusion, falls and anticholinergic effects; use lower doses and monitor closely.
• Report signs of urinary retention, severe dry mouth, dilated pupils with blurred vision, high fever, tremor or hallucinations–these may indicate significant anticholinergic toxicity requiring urgent care.

Using Promethazine as a Short-Term Sedative: Onset, Duration, and Monitoring Requirements

Administer promethazine 25–50 mg orally or intramuscularly for short-term sedation; expect noticeable sedation within 15–60 minutes and plan for clinical effects to last roughly 4–6 hours for most adults.

Onset and duration by route

Route	Typical sedative dose (adult)	Onset	Peak	Usual duration
Oral (PO)	25–50 mg	20–60 min	~1–2 hr	4–6 hr (residual drowsiness up to 12 hr)
Intramuscular (IM)	25–50 mg	15–30 min	~30–60 min	4–6 hr
Intravenous (IV)	Clinician-only use; dilute and give slowly	Immediate–5 min	Minutes	3–4 hr
Rectal	25–50 mg	20–40 min	~1 hr	4–6 hr

Adjust doses downward for older adults (start 12.5–25 mg) and for patients with hepatic impairment; do not exceed 100 mg/day in adults. Avoid routine use in children under 2 years; if used in older children, use weight-based dosing (commonly 0.25–0.5 mg/kg per dose, single dose usually ≤25 mg) and monitor closely for respiratory depression.

Monitoring and safety checklist

Before dosing: confirm no hypersensitivity to phenothiazines, no recent MAOI use (avoid within 14 days), and review concomitant CNS depressants (opioids, benzodiazepines, alcohol). Obtain baseline respiratory rate, oxygen saturation, blood pressure, and level of consciousness.

During and after dosing: monitor vitals every 15 minutes for the first hour, then every 30–60 minutes until the patient returns to baseline alertness. Observe respiratory rate and oxygen saturation continuously if IV or if the patient receives other sedatives. Keep the patient seated or supervised when mobilizing until gait and cognition return to baseline.

Discharge criteria: stable respiratory rate and oxygen saturation on room air, systolic blood pressure within patient’s acceptable range, alert and oriented to person/place/time, able to ambulate or transfer safely, and no new dysphoria or severe dizziness. Advise against driving or operating machinery for 24 hours and instruct to avoid alcohol and other sedatives for the same period.

Managing adverse effects: for excessive sedation or respiratory depression, secure airway, provide supplemental oxygen, support ventilation, and call emergency services if needed. Treat hypotension with IV fluids and vasopressors as clinically indicated. For acute dystonic reactions, administer an anticholinergic (e.g., benztropine) or diphenhydramine. Activated charcoal may be considered within 1 hour of recent oral overdose if the airway is protected.

Document dose, route, time, monitoring intervals, and patient response. Increase observation duration for elderly patients, those with obstructive sleep apnea, chronic lung disease, or concurrent opioid/benzodiazepine use. Consider alternative agents if high fall risk, severe cardiovascular disease, or baseline prolonged QT interval exist.

Pediatric Use, Age Limits, and Safer Alternatives to Promethazine in Children

Do not give promethazine to children younger than 2 years of age – the U.S. FDA specifically contraindicates use in this age group because of risk of severe respiratory depression and death.

For children aged 2 years and older, restrict promethazine to situations where a pediatric prescriber judges no safer alternative exists. If prescribed, use the lowest effective weight-based dose and supervise breathing and level of alertness for at least several hours after dosing. Typical prescriber guidance uses a weight-based range (about 0.25–1 mg/kg per dose) with a common maximum single dose of 25 mg, but exact dosing and maximum daily limits must come from the child’s clinician and product labeling. Avoid combining promethazine with other central nervous system depressants (opioids, benzodiazepines, alcohol) and do not use it as a routine cough/cold medicine or as a sedative for routine procedures or sleep.

Safer alternatives for nausea and vomiting

Prefer ondansetron over promethazine for pediatric nausea and vomiting. Typical dosing: ondansetron 0.15 mg/kg orally or IV per dose (common maximum single pediatric dose 4 mg); repeat dosing and route depend on clinical setting and clinician instructions. Use oral rehydration for gastroenteritis; reserve antiemetics for ongoing vomiting that prevents oral intake or for chemotherapy-directed protocols under oncology guidance.

Safer alternatives for allergic symptoms and motion sickness

Use second-generation antihistamines for allergic rhinitis or chronic urticaria because they produce less sedation and respiratory depression. Weight-based estimates commonly used by clinicians: cetirizine ~0.25 mg/kg once daily (typical maximum 10 mg/day) and loratadine ~0.2 mg/kg once daily (typical maximum 10 mg/day); follow product labeling or pediatric advice for exact dosing by age and formulation. For acute urticaria or severe allergic reaction, give diphenhydramine (antihistamine) per pediatric dosing only after epinephrine when anaphylaxis is suspected. For motion sickness in older children, consider non‑sedating strategies first (behavioral adjustments, seating position, adequate ventilation); pharmacologic options such as dimenhydrinate or meclizine have age restrictions and should be used per product labeling and pediatrician recommendation.

Seek emergency care if a child given promethazine develops slow or shallow breathing, marked drowsiness or unresponsiveness, cyanosis, persistent vomiting causing dehydration, or any signs of severe allergic reaction (wheezing, swelling of face or throat). Always check the prescribing information and consult a pediatrician or pharmacist before administering any antiemetic or antihistamine to a child.

Contraindications and Major Drug Interactions with Promethazine: What to Stop or Avoid

Stop alcohol and other sedating agents while taking promethazine; avoid combining it with opioids, benzodiazepines, barbiturates, sedating antihistamines, or other CNS depressants unless dose reduction and close respiratory monitoring are arranged.

Do not use promethazine in patients with known hypersensitivity to promethazine or other phenothiazines, in comatose states, or for treatment of lower respiratory tract symptoms (including acute asthma exacerbations). Avoid use in patients with a history of severe hepatic impairment or acute porphyria unless a specialist approves therapy.

Avoid concurrent use with monoamine oxidase inhibitors (MAOIs). Allow a 14-day MAOI washout before starting promethazine when possible; if MAOI therapy cannot be stopped, choose an alternative antiemetic/antihistamine that has documented safety with MAOIs.

Stop or reduce doses of opioids and other respiratory depressants when adding promethazine. If unavoidable, reduce the opioid dose, use the lowest effective promethazine dose, and monitor respiratory rate, oxygenation, and level of consciousness. Expect amplified sedation and possible life‑threatening respiratory depression with combinations such as promethazine + morphine, oxycodone, methadone, or fentanyl.

Avoid combining promethazine with other drugs that prolong the QT interval (examples: amiodarone, sotalol, quinidine, procainamide, macrolide antibiotics such as erythromycin and clarithromycin, and certain fluoroquinolones). If coadministration cannot be avoided, obtain baseline ECG, correct hypokalemia/hypomagnesemia, and repeat ECG monitoring during initiation and dose changes.

Avoid or use caution with strong CYP2D6 inhibitors (for example, fluoxetine, paroxetine, quinidine) because they may raise promethazine plasma levels and intensify adverse effects; consider dose reduction and clinical monitoring for excess sedation, anticholinergic effects, or hypotension.

Avoid combining promethazine with other anticholinergic agents (such as oxybutynin, tolterodine, ipratropium) because additive anticholinergic effects can cause severe urinary retention, acute narrow‑angle glaucoma, hyperthermia, and severe constipation; stop the nonessential anticholinergic when initiating promethazine.

Do not coadminister promethazine with other dopamine antagonists (metoclopramide, other phenothiazines, certain antipsychotics) unless you monitor closely for extrapyramidal symptoms and neuroleptic malignant syndrome; if EPS develops, stop promethazine and treat per neurology/psychiatry guidance.

Antihypertensives and nitrates: avoid routine coadministration without monitoring. Promethazine can enhance orthostatic hypotension; lower blood pressure or syncope may occur. Check orthostatic vital signs after dose changes and adjust antihypertensive therapy as needed.

In patients with seizure disorders, avoid adding promethazine to drugs that lower seizure threshold (bupropion, tramadol, high‑dose clozapine) and monitor seizure control; consider alternative antiemetics for those with unstable seizure disorders.

If severe sedation, respiratory depression, new or worsening arrhythmia, acute urinary retention, signs of acute glaucoma, or marked hypotension appear, discontinue promethazine immediately and arrange clinical assessment and supportive care.