Phenergan (Promethazine) Tablet Uses and Dosing
Phenergan tablets are FDA-approved for treating allergic conditions, motion sickness, nausea/vomiting, and sedation in adults and children over 2 years of age, with typical adult dosing of 12.5-25 mg depending on the indication. 1
Primary Indications and Dosing
Allergic Conditions
- Standard dose: 25 mg at bedtime, or 12.5 mg before meals and at bedtime if necessary 1
- Alternative regimen: 6.25-12.5 mg three times daily for maintenance 1
- Transfusion reactions: 25 mg controls minor allergic transfusion reactions 1
Motion Sickness
- Adult dose: 25 mg twice daily 1
- Timing: Initial dose 30-60 minutes before travel, repeat 8-12 hours later 1
- Subsequent days: 25 mg upon arising and before evening meal 1
- Pediatric dose: 12.5-25 mg twice daily for children over 2 years 1
Nausea and Vomiting
- Active treatment: 25 mg as the effective dose for adults 1
- Repeat dosing: 12.5-25 mg every 4-6 hours as needed 1
- Pediatric dosing: 0.5 mg per pound of body weight, adjusted for age and severity 1
- Prophylaxis (surgical): 25 mg repeated at 4-6 hour intervals 1
Sedation and Insomnia
- Nighttime sedation: 25-50 mg at bedtime for adults 1
- Pediatric sedation: 12.5-25 mg at bedtime for children over 2 years 1
- Preoperative use: 50 mg the night before surgery for adults 1
Critical Safety Considerations
Absolute Contraindications
- Children under 2 years: Fatal respiratory depression risk—this is an absolute contraindication 2, 1
- Concurrent MAOI use: Contraindicated due to dangerous drug interactions 2
High-Risk Populations Requiring Caution
- Closed-angle glaucoma, prostatic hypertrophy, bladder neck obstruction: Anticholinergic effects can worsen these conditions 2
- Hyperthyroidism: Potential cardiovascular effects require monitoring 2
- Elderly patients: Consider lower starting doses (6.25 mg) due to increased sensitivity and fewer adverse effects at this dose 3
Evidence-Based Dosing Modifications
Lower-Dose Strategy for Reduced Side Effects
Research demonstrates that 6.25 mg IV promethazine provides equivalent antiemetic efficacy to standard 25 mg dosing with significantly fewer adverse effects 4, 3. While these studies used IV administration, the principle of using the lowest effective dose applies to oral therapy as well 1.
- 6.25 mg achieved 74% relief at 1 hour versus 59% for ondansetron 4 mg 4
- In elderly patients, 6.25 mg had significantly fewer adverse drug reactions than 12.5 mg (p=0.048) 3
- Time to relief was equivalent between 6.25 mg and 12.5 mg doses 3
Route-Specific Considerations
- Oral bioavailability is only 25%, yet dosing references suggest identical doses regardless of route—this supports starting with lower oral doses when appropriate 4
- IV administration carries risk of thrombophlebitis and tissue necrosis; deep intramuscular injection is the preferred parenteral route 5
Common Pitfalls and How to Avoid Them
Excessive Sedation
- Promethazine causes more sedation than alternative antiemetics like ondansetron 6
- Start with 12.5 mg rather than 25 mg when sedation is undesirable 1
- Avoid combining with narcotic analgesics without dose reduction 1, 4
Pediatric Dosing Errors
- Never use in children under 2 years—this is a black box warning 2, 1
- Always calculate pediatric doses by weight (0.5 mg/lb) rather than using adult doses 1
- Adjust to the smallest effective amount after initiating treatment 1
Drug Interactions
- Avoid concurrent use with MAOIs 2
- Exercise caution when combining with other sedating medications 4
- Monitor for enhanced anticholinergic effects with other anticholinergic drugs 2
Alternative Considerations
When sedation is problematic or contraindications exist, ondansetron may be preferred as it provides equivalent antiemetic efficacy without sedation or akathisia risk 6. However, promethazine remains appropriate when sedation is actually desirable or cost is a limiting factor 6.