What is the recommended dose and frequency of Phenergan (promethazine) for adults?

Promethazine (Phenergan) Dosing and Frequency for Adults

The recommended dose of promethazine for adults is 12.5-25 mg every 4-6 hours as needed, with the specific dosage depending on the indication being treated. 1

Dosing by Indication

Nausea and Vomiting

• Standard dose: 25 mg every 4-6 hours as needed 1
• Lower doses of 12.5 mg may be used initially and repeated as necessary 1
• For prophylaxis of nausea and vomiting (e.g., during surgery or postoperative period): 25 mg repeated at 4-6 hour intervals 1

Allergy

• Standard dose: 25 mg taken before retiring 1
• Alternative regimen: 12.5 mg taken before meals and on retiring, if necessary 1
• Single 25 mg doses at bedtime or 6.25-12.5 mg three times daily is usually sufficient 1
• After treatment initiation, dosage should be adjusted to the smallest amount adequate to relieve symptoms 1

Motion Sickness

• Standard dose: 25 mg taken twice daily 1
• Initial dose should be taken 30-60 minutes before anticipated travel
• Repeat dose 8-12 hours later if necessary
• On subsequent travel days: 25 mg on arising and again before the evening meal 1

Sedation

• Standard dose: 25-50 mg at bedtime for nighttime, presurgical, or obstetrical sedation 1
• For pre- and postoperative use: 25-50 mg with appropriately reduced doses of narcotics or barbiturates 1

Administration Considerations

• Promethazine can be administered orally, parenterally, or rectally when oral medication cannot be tolerated 1
• For intravenous administration, peripheral IV administration can cause tissue injury, including gangrene or thrombophlebitis 2
• When used for cyclic vomiting syndrome, promethazine can be given 12.5-25 mg by mouth or per rectum every 4-6 hours during episodes 2

Side Effects and Precautions

Common Side Effects

• CNS depression (sedation is common) 2
• Anticholinergic effects 2
• Extrapyramidal symptoms 2

Special Precautions

• Contraindicated in children under 2 years of age 1
• Use with caution in patients with glaucoma, benign prostatic hypertrophy, ischemic heart disease, or hypertension due to anticholinergic effects 2
• May cause oversedation and confusion, particularly in older adults 2

Comparative Efficacy

Studies have shown that prochlorperazine (10 mg IV) may be more effective than promethazine (25 mg IV) for treating uncomplicated nausea and vomiting in emergency department settings, with fewer complaints of sleepiness (38% vs 71%) 3. However, promethazine remains a valuable option when sedation is desirable 4.

Clinical Pearls

• Promethazine has minimal effects on autonomic cardiovascular mechanisms, making it suitable for patients where cardiovascular stability is important 5
• For breakthrough nausea and vomiting during chemotherapy, promethazine can be used at 12.5-25 mg PO or IV (central line only) every 4 hours 2
• When used for pruritus management in cancer pain, promethazine can be administered at 12.5-25 mg PO every 6 hours 2

Remember that dosage should always be adjusted to the smallest amount adequate to relieve symptoms after initiation of treatment 1.