Medication Overview

Phenergan, also known as promethazine, is a phenothiazine derivative with antihistaminic, sedative, antiemetic, and anticholinergic properties. It is utilized to treat a variety of conditions, including allergies, motion sickness, nausea and vomiting, and as a sedative or sleep aid.

Pharmacology and Mechanism

Phenergan functions primarily by blocking the histamine H1 receptors, providing relief from allergic reactions. It has significant sedative effects due to its action on the central nervous system, where it inhibits histamine and muscarinic receptors. This also contributes to its antiemetic and anticholinergic properties.

Indications for Use

Phenergan is indicated for the treatment of allergic conditions such as rhinitis, conjunctivitis, and dermographism. It is also used to manage motion sickness, to alleviate nausea and vomiting, including post-operative nausea, and as a sedative for preoperative or postoperative patients.

Dosage Forms and Strengths

Phenergan is available in multiple forms including oral tablets, oral syrup, rectal suppositories, and injectable solutions. The strengths commonly available are 12.5 mg, 25 mg, and 50 mg tablets, as well as 6.25 mg/5 mL syrup and 25 mg rectal suppositories. Injectable doses are typically offered in 25 mg/mL and 50 mg/mL concentrations.

Administration Guidelines

Oral dosage forms should be taken with or without food, according to individual tolerance. Suppositories are inserted rectally, approximately 30 minutes before experiencing symptoms of motion sickness. Intramuscular injections should be administered in a large muscle mass, while intravenous forms must be diluted and administered slowly to avoid complications.

Pharmacokinetics

Following oral administration, Phenergan is rapidly absorbed with an onset of action within 20 minutes. Peak plasma levels are typically reached within 2-3 hours. The medication is extensively metabolized in the liver, and its elimination half-life ranges from 10 to 14 hours, with excretion primarily via renal pathways.

Potential Drug Interactions

Concurrent use of central nervous system depressants, such as alcohol, opioids, and benzodiazepines, can enhance the sedative effects of Phenergan. It may potentiate antihypertensive effects of MAO inhibitors and beta-blockers. Co-administration with anticholinergic drugs can increase the risk of anticholinergic side effects.

Contraindications

Phenergan is contraindicated in individuals with hypersensitivity to promethazine or other phenothiazines. Other contraindications include patients with severe respiratory depression, comatose states, and for use in children under two years of age due to risk of fatal respiratory depression. It should not be used in patients who have ingested large amounts of CNS depressants.

Adverse Reactions

Common side effects include drowsiness, dizziness, dry mouth, blurred vision, and constipation. Severe reactions may involve respiratory depression, severe hypotension, neuroleptic malignant syndrome, seizures, or severe tissue injury with parenteral use. Cardiovascular adverse effects, such as QT prolongation and ventricular arrhythmias, have also been reported.

Monitoring Parameters

Patients should be monitored for signs of excessive sedation, respiratory depression, and anticholinergic effects such as urinary retention or severe constipation. Periodic assessment of liver and kidney function may be warranted. Monitoring for cardiovascular signs such as prolonged QT interval is advisable, especially in patients with existing heart conditions.

Special Populations Considerations

In elderly individuals, lower initial doses should be utilized due to increased sensitivity to sedative and anticholinergic effects. Caution is advised in patients with glaucoma, enlarged prostate, or respiratory disorders due to the additive anticholinergic effects. Dosing adjustments may be necessary for those with hepatic or renal impairments.

Phenergan and Pregnancy

Phenergan falls under Pregnancy Category C. Studies in animals have shown adverse effects on the fetus, and there are no well-controlled studies in pregnant women. It should be prescribed during pregnancy only if the potential benefit justifies the potential risk to the fetus. Phenergan should be used with caution during labor and delivery, considering its sedative effects.

Storage and Handling

Phenergan tablets and syrup should be stored at controlled room temperature, between 68°F to 77°F (20°C to 25°C). Suppositories are to be stored in a refrigerator and protected from light. Injectable solutions should be stored in a tightly sealed container, protected from light, and discarded if they become discolored or particulate matter is observed.