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- 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 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 brand names are available for promethazine?
Phenergan, Phenadoz, Promethegan
Is promethazine available as a generic drug?
GENERIC AVAILABLE: Yes
Do I need a prescription for promethazine?
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:
- missed menstrual periods,
- breast enlargement or tenderness,
- loss of sexual drive,
- inability to ejaculate, and
- priapism, and
- Weight gain.
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