What is promethazine, and how does it work (mechanism of action)?
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.
What are the side effects of promethazine?
Promethazine causes sedation, confusion, and disorientation. 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.
Other side effects include anticholinergic side effects such as:
- blurred vision,
- dry mouth,
- dilated pupils,
- urinary retention (inability to urinate),
- impotence, and
EPS may occur. EPS are categorized as dystonic reactions (alterations in muscle tone), sharp, involuntary muscle movements (often limited to one muscle or muscle group), akathisia (subjective restlessness), and Parkinsonism. Parkinsonian symptoms are more common in older persons whereas children more often develop involuntary muscle movement reactions. Dystonic reactions are most commonly seen during the first week of treatment. Restlessness and Parkinsonian symptoms usually develop days to weeks after starting therapy.
A serious complex called neuroleptic malignant syndrome (NMS) can occur in patients receiving phenothiazines. NMS consists of high body temperature, severe EPS, changes in consciousness and mental status, and increased heart rate with low or high blood pressure. NMS occurs more frequently in young men and in persons who are dehydrated.
Rarely, blood cell disorders can occur; low white cell counts can lead to infections.
Phenothiazines such as promethazine can cause skin hyperpigmentation (darkening) but usually only after prolonged use. The effect usually is restricted to areas of the body exposed to sunlight. Thus, people who need long-term treatment with promethazine should either keep out of the sun or use effective sunscreens.
Phenothiazines can cause blurred vision, difficulty with nighttime vision, or changes in color vision.
Phenothiazines such as promethazine block dopamine receptors. This effect can lead to increases in blood levels of prolactin, a hormone involved in lactation (formation of breast milk). As a result, phenothiazines can cause the breast to produce fluid ("milk") even when a woman is not pregnant.
Additionally, phenothiazines can cause:
- 10 Common Allergy Triggers Slideshow Pictures
- Take the Quiz on Allergies
- Nasal Allergy Relief Products Slideshow
- What is promethazine, and how does it work (mechanism of action)?
- What brand names are available for promethazine?
- Is promethazine available as a generic drug?
- Do I need a prescription for promethazine?
- What are the side effects of promethazine?
- What is the dosage for promethazine?
- Is promethazine safe to take if I'm pregnant or breastfeeding?
- What else should I know about promethazine?
What is the dosage for promethazine?
- Allergic reactions are treated with 6.25-25 mg orally 3 times daily. A single 25 mg dose administered at bedtime also may suffice. A 25 mg injection is also used.
- Nausea and vomiting may be managed with 12.5-25 mg administered orally by injection every 4-6 hours as needed.
- Doses of 25-50 mg by injection are used for sedation before or after surgery.
- 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.
Is promethazine safe to take if I'm pregnant or breastfeeding?
There are no adequate studies of promethazine in pregnant women. Administration of promethazine within two weeks of delivery may affect platelet function in the newborn.
It is not known whether promethazine is excreted in breast milk.
What else should I know about promethazine?
What preparations of promethazine are available?
- 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.
How should I keep promethazine stored?
Tablets, syrup and injection should be stored at room temperature, between 15 C to 25 C (68 F to 77 F). Suppositories should be stored at 2 C to 8 C (36 F to 46 F).
Promethazine (Phenergan [Discontinued brand], Phenadoz, Promethegan) is a drug prescribed to treat nausea, vomiting, motion sickness, allergic reactions, and for sedation prior to surgery. Side effects and numerous drug interactions should be reviewed prior to taking this medication.
Multimedia: Slideshows, Images & Quizzes
Related Disease Conditions
Hay Fever (Allergic Rhinitis)
Hay fever (allergic rhinitis) is an irritation of the nose caused by pollen and is associated with the following allergic...
Latex allergy is a condition where the body reacts to latex, a natural product derived from the rubber tree. The reaction can...
Meniere disease (idiopathic endolymphatic hydrops) is an inner ear disorder with symptoms that include: vertigo, tinnitus,...
Insect Sting Allergies
The majority of stinging insects in the United States are from bees, yellow jackets, hornets, wasps, and fire ants. Severity of...
Vertigo (Symptoms, Causes, Treatments, and Home Remedies)
Vertigo is the sensation of spinning or rocking, even when someone is at rest. Vertigo may be caused by a problem in the brain or...
An allergy refers to a misguided reaction by our immune system in response to bodily contact with certain foreign substances. ...
Motion sickness is a feeling of unwellness caused by the inner ear and balance systems. Motion sickness can include sea...
Sun-Sensitive Drugs (Photosensitivity to Drugs)
Sun sensitivity (photosensitivity) is an inflammation of the skin induced by the combination of medications or substances and...
Nausea and vomiting after eating are symptoms that may be caused by many conditions. Antiemetics are drugs that help get rid of...
The allergic cascade refers to allergic reactions that happen in the body in response to allergens. A variety of immune cells and...
Nausea and Vomiting (Causes, Natural Remedies, Diet, Medication)
Nausea is an uneasiness of the stomach that often precedes vomiting. Nausea and vomiting are not diseases, but they are symptoms...
Balance is a state of body equilibrium or stability. We often take for granted how dependent we are on a healthy balance system....
Labyrinthitis (Inner Ear Inflammation) Symptoms, Causes, Contagious, Treatment Cure
Labyrinthitis is inflammation of the labyrinth (the part of the ear responsible for balance and hearing). Doctors do not know...
Inner Ear Infection (Symptoms, Signs, Treatments, Home Remedies)
An inner ear infection or otitis interna is caused by viruses or bacteria and can occur in both adults and children. An inner...
Treatment & Diagnosis
- Vertigo Balance Disorders FAQs
- Drugs: Buying Prescription Drugs Online Safely
- Medication Disposal
- Dangers of Mixing Medications
- How To Reduce Your Medication Costs
- Pharmacy Visit, How To Get The Most Out of Your Visit
- Indications for Drugs: Approved vs. Non-approved
- Drugs: The Most Common Medication Errors
- Generic Drugs, Are They as Good as Brand-Names?
Medications & Supplements
Prevention & Wellness
Health Solutions From Our Sponsors
Report Problems to the Food and Drug Administration
You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.