Periactin
Meaning of Periactin: An allergy tablet kept in use largely for its side effects, from appetite gain to reversing a dangerous serotonin excess.
Definition of Periactin
Periactin is a trade name for cyproheptadine hydrochloride, an older antihistamine taken by mouth for allergic itching, hives and hay fever. It blocks histamine, the chemical released during an allergic reaction, and it also blocks serotonin, a separate messenger. That second action explains its unusual secondary uses, which include stimulating appetite and heading off migraine attacks.
What does it actually block?
Three things at once, which is typical of drugs from its era. It occupies histamine receptors, stopping the itching, sneezing, streaming and welts of an allergic reaction. It occupies serotonin receptors, which histamine has nothing to do with. And it partly blocks acetylcholine, another messenger, which is where the dry mouth and blurred vision come from. Unlike the newer allergy tablets it crosses readily into the brain, so drowsiness is the rule rather than the exception, and that sedation is sometimes the reason it is chosen at night.
Why is it still used when newer antihistamines exist?
| Use | Which action explains it |
|---|---|
| Itching, hives and hay fever | Histamine blockade |
| Poor appetite and weight gain | Serotonin blockade in appetite centers |
| Preventing migraine in children | Serotonin blockade |
| Serotonin syndrome | Serotonin blockade, used as an antidote |
The last of those is the most striking. Serotonin syndrome is a dangerous state of agitation, fever, tremor and muscle rigidity caused by too much serotonin activity, usually from a combination of medicines, and this antihistamine is the agent given to oppose it. Nothing about its allergy role would suggest such a use.
What are the drawbacks?
Drowsiness, dry mouth, blurred vision, constipation and difficulty passing urine, the last more troublesome where the prostate is already enlarged. In older people the same properties can cause confusion and unsteadiness, which is why sedating antihistamines appear on lists of medicines used with caution later in life. For plain hay fever it has largely given way to newer tablets that do not cause drowsiness. The generic name cyproheptadine is now used more often than the brand, and a related sedating antihistamine, promethazine hydrochloride, shares much of the same profile.
Also known as
Used in a sentence
Periactin was prescribed at night for persistent itching.
Good to know
Cyproheptadine blocks serotonin as well as histamine, which is why an allergy medicine serves as the antidote to serotonin syndrome and as an appetite stimulant.