promethazine hydrochloride
Meaning of promethazine hydrochloride: An old antihistamine kept in use less for allergy than for the two side effects it produces reliably: sleepiness and the settling of nausea.
Definition of promethazine hydrochloride
Promethazine hydrochloride is the salt form of promethazine, an older antihistamine built on a phenothiazine backbone. It blocks histamine H1 receptors, and it also blocks receptors in the brain that trigger vomiting, which is why it is used far more often for nausea, motion sickness and sedation than for allergic symptoms. It is available as tablets, syrup, suppositories and injection.
Why does one antihistamine do so many different things?
Because it is not selective. Promethazine reaches the brain freely, unlike the newer antihistamines designed to stay out of it, and once there it blocks histamine receptors that help maintain wakefulness. That produces the marked drowsiness which is the drug's most noticeable effect. It also blocks dopamine and muscarinic receptors, and the dopamine blockade in the vomiting trigger zone of the brainstem is what gives it an antiemetic action quite separate from anything to do with allergy.
What is it used for?
| Use | Which action is responsible |
|---|---|
| Nausea and vomiting | Dopamine and histamine blockade in the brainstem |
| Motion sickness | Anticholinergic action on the balance pathways |
| Short-term sedation | Central histamine blockade |
| Allergic reactions and itch | Peripheral blockade of histamine at H1 receptors |
What does the phenothiazine backbone imply?
It is the same chemical family as the first antipsychotic drugs, and the shared structure explains a shared set of problems. Movement disorders can occur, including restlessness and, rarely, sustained muscle spasms of the neck and eyes. The drug lowers the seizure threshold and can drop blood pressure when given quickly into a vein. None of that is expected from a simple allergy medicine, and it is the reason a drug that looks mild on paper carries the cautions it does.
Why is the injectable form treated with such care?
Promethazine is intensely irritating to tissue. If it escapes from a vein, or is injected into an artery by mistake, it can destroy the surrounding tissue and has led to loss of fingers and limbs. It is also markedly more dangerous in very young children, in whom it has caused fatal breathing depression, and it is not used in that group. Its sedative effect adds to that of alcohol and opioids.
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Used in a sentence
Promethazine hydrochloride was given for persistent nausea after the procedure.
Good to know
The drug is a phenothiazine, so it shares the movement disorder and blood pressure effects of that family, and its injectable form causes severe tissue damage if it leaks outside a vein.