Promethazine DM Dosing
For adults, the recommended dose of promethazine-dextromethorphan (Promethazine DM) oral solution is 5 mL (containing 6.25 mg promethazine and 15 mg dextromethorphan) every 4–6 hours, not exceeding 30 mL in 24 hours. 1
Pediatric Dosing by Age
The FDA-approved dosing for Promethazine DM oral solution varies by age group and is contraindicated in children under 2 years of age due to the risk of fatal respiratory depression: 1
- Children 2 to under 6 years: 1.25–2.5 mL every 4–6 hours, not exceeding 10 mL in 24 hours 1
- Children 6 to under 12 years: 2.5–5 mL every 4–6 hours, not exceeding 20 mL in 24 hours 1
- Children 12 years and older (adolescents): 5 mL every 4–6 hours, not exceeding 30 mL in 24 hours 1
Critical Safety Warnings
Black Box Warning for Pediatric Use
Promethazine is absolutely contraindicated in children under 2 years of age because postmarketing reports have documented fatal respiratory depression in this population. 1 Even when used in children 2 years and older, the lowest effective dose should be employed, and concomitant administration of other respiratory depressants (opioids, benzodiazepines, alcohol) must be avoided. 1
Respiratory Depression Risk
Promethazine may cause potentially fatal respiratory depression and should be avoided in patients with compromised respiratory function, including those with COPD or sleep apnea. 1 The sedative and respiratory depressant effects are amplified when combined with other CNS depressants such as alcohol, narcotics, benzodiazepines, or barbiturates. 1
Administration Precautions
When promethazine is given intravenously (as a single agent, not in the DM combination), it must be infused slowly at no more than 25 mg/min to minimize hypotension risk. 2, 3 The usual IV dose for promethazine alone is 12.5–25 mg, with a total dose range of 25–50 mg when used as an adjuvant. 3 However, the Promethazine DM oral solution formulation discussed here is for oral use only. 1
Recent safety alerts emphasize that injectable promethazine carries rare but serious risks of limb-threatening complications from extravasation or inadvertent intra-arterial injection, and intramuscular administration is now preferred over intravenous when parenteral dosing is necessary. 4
Dosing Considerations for Specific Populations
Elderly Patients
In geriatric patients, promethazine should be started at the low end of the dosing range (6.25 mg for antiemetic use) because sedating drugs may cause confusion and over-sedation in the elderly. 1 Research demonstrates that 6.25 mg IV promethazine is as effective as 12.5 mg for treating nausea and vomiting in hospitalized elderly patients, with significantly fewer adverse drug reactions. 5
Pregnancy
When promethazine is used for nausea and vomiting in pregnancy (as a single agent, not combined with dextromethorphan), the recommended dose is 12.5–25 mg every 4–8 hours via oral, intramuscular, or intravenous routes. 6 Promethazine is classified as Pregnancy Category C and should only be used when potential benefits justify fetal risks. 1
Drug Interactions
Promethazine DM is absolutely contraindicated with monoamine oxidase inhibitors (MAOIs) because dextromethorphan combined with MAOIs has caused hyperpyrexia, hypotension, and death. 1 When promethazine is given with narcotics, the narcotic dose should be reduced by one-quarter to one-half; when given with barbiturates, the barbiturate dose should be reduced by at least one-half. 1
Mechanism and Pharmacokinetics
Promethazine is a first-generation phenothiazine antihistamine with H1-receptor antagonist, sedative, antiemetic, and anticholinergic properties. 2, 3 Clinical effects appear within 5 minutes of IV administration, with a duration of action of 4–6 hours and a plasma half-life of 9–16 hours. 2, 3 Dextromethorphan is a non-sedating opioid derivative that acts centrally to suppress the cough reflex. 7, 8
Common Pitfalls to Avoid
- Never use promethazine in children under 2 years of age under any circumstances 1
- Do not combine with MAOIs due to life-threatening interactions with dextromethorphan 1
- Avoid combining with other CNS depressants without dose reduction, as additive sedation and respiratory depression can occur 1
- Do not use epinephrine to treat promethazine-associated hypotension, as promethazine may reverse epinephrine's vasopressor effect 1
- Counsel patients to avoid driving or operating machinery until they know how the medication affects them, as promethazine impairs mental and physical abilities 1