Pharmacy Author: Omudhome Ogbru, PharmD
Medical and Pharmacy Editor: Jay W. Marks, MD

GENERIC NAME: promethazine

BRAND NAME: Phenergan, Phenadoz, Promethegan

DRUG CLASS AND MECHANISM: Promethazine is a phenothiazine in the same drug class as chlorpromazine (Thorazine) and trifluoperazine (Stelazine). However, unlike the other drugs in this class, promethazine is not used as an anti-psychotic. It used as an anti-histamine, sedative, and antiemetic (anti-nausea). The body releases histamine during several types of allergic reactions. When histamine binds to its receptors on cells, it stimulates changes within the cells that lead to sneezing, itching, and increased mucus production. Antihistamines such as promethazine compete with histamine for one of the receptors for histamine (the H1 receptor) on cells. However, when the antihistamines bind to the receptors they do not stimulate the cells. Instead, they prevent histamine from binding and stimulating the cells. Promethazine also blocks the action of acetylcholine (anticholinergic effect), and this may explain its benefit in reducing the nausea of motion sickness. It is used as a sedative because it causes drowsiness as a side effect. The FDA approved promethazine in 1951.

PRESCRIPTION: Yes

GENERIC AVAILABLE: Yes

PREPARATIONS: Tablets: 12.5, 25, and 50 mg; Suppositories: 12.5, 25, and 50 mg; Syrup: 6.25 mg/5 ml; Injection: 25 and 50 mg/ml.

STORAGE: Tablets, syrup and injection should be stored at room temperature between 15-25 C (68-77 F). Suppositories should be stored at 2-8 C (36-46 F).

PRESCRIBED FOR: Promethazine is prescribed for treating nausea or vomiting, motion sickness, and allergic reactions and for sedation.

DOSING:

• Allergic reactions are treated with 6.25-25 mg 3 times daily. A single 25 mg dose administered at bedtime also may suffice.
  
  
• Nausea and vomiting may be managed with 12.5-25 mg administered every 4-6 hours as needed.
  
  
• Doses of 12.5-50 mg at bedtime are used for sedation.
  
  
• For prevention of motion sickness, 25 mg is used 30 to 60 minutes before the motion begins and then every 8 to 12 hours as needed. Oral, rectal and injectable doses are similar.
  
  
• Promethazine injections are used when the oral route is not possible (for example, with severe vomiting).
  
  
• Tablets may be taken with or without food.
  
  
• Suppositories are unwrapped and moistened with water before insertion into the rectum. If the suppository is too soft from being stored in a warm place, it may be chilled in the refrigerator for 20 to 30 minutes or placed in cold water before the wrapper is removed.

DRUG INTERACTIONS: Promethazine should not be taken with any of the MAO (mono-amine oxidase) inhibitor-class of antidepressants, for example, isocarboxazid (Marplan), phenelzine (Nardil), tranylcypromine (Parnate), and procarbazine (Matulane), because of the increased risk of extrapyramidal symptoms (EPS)--uncontrollable movement disorders.

Excessive anti-cholinergic effects (described below) can occur when promethazine is used with other antihistamines, for example, diphenhydramine (Benadryl); some phenothiazines, for example, thioridazine (Mellaril); some tricyclic antidepressants, for example, amitriptyline (Elavil); clozapine (Clozaril), cyclobenzaprine (Flexeril), and disopyramide (Norpace).

Excessive sedation may occur when promethazine is combined with other medications that depress the central nervous system (brain) and cause sedation. Such drugs include ethanol, barbiturates, anti-anxiety medications, sedatives, other phenothiazines, and narcotic pain medications.

There may be an increase in the risk of certain neurologic reactions that affect movement of muscles (EPS, see below) when promethazine is combined with medicines that also cause EPS. Such drugs include antipsychotics, metoclopramide (Reglan), and amoxapine (Asendin).

Promethazine should not be used with propylthiouracil (PTU) due to the increased risk of low white blood cell counts and increased risk of infections. The reason for this interaction is not known.

Concurrent use of promethazine with the dye used for myelography (x-rays of the spinal cord) can lower the threshold for seizures and thus increase the risk of seizures. Promethazine should be stopped at least 48 hours before myelography and not restarted until at least 24 hours after myelography.

PREGNANCY: There are no adequate studies of promethazine in pregnant women. Administration of promethazine within 2 weeks of delivery may affect platelet function in the newborn.

NURSING MOTHERS: It is not known whether promethazine is excreted in breast-milk.

SIDE EFFECTS: Promethazine often causes sedation. In children less than two it can depress respiration and lead to death. Therefore, it should not be used in children less than two years old. Dizziness may also occur. Ironically, promethazine sometimes stimulates patients, particularly children. Such stimulation may be manifest by restlessness, inability to sleep, heart palpitations or even seizures. Promethazine also causes anticholinergic side effects such as blurred vision, dry mouth, dilated pupils, nausea, urinary retention (inability to urinate), impotence, and constipation.