Doxylamine

Doxylamine is an antihistamine medication used to treat insomnia and allergies, and—in combination with pyridoxine (vitamin B₆)—to treat morning sickness in pregnant women. It is available over-the-counter and is sold under such brand names as Equate or Unisom, among others; and it is used in nighttime cold medicines (e.g., NyQuil) and pain medications containing paracetamol (acetaminophen) or codeine to help with sleep. The medication is delivered chemically by the salt doxylamine succinate and is taken by mouth. Doxylamine and other first-generation antihistamines are the most widely used sleep medications in the world. Typical side effects of doxylamine include dizziness, drowsiness, grogginess, and dry mouth, among others.

As an antihistamine, doxylamine is an inverse agonist of the histamine H₁ receptor. As a first-generation antihistamine, it typically crosses the blood–brain barrier into the brain, thereby producing a suite of sedative and hypnotic effects that are mediated by the central nervous system.

Doxylamine is also a potent anticholinergic, meaning that it causes delirum at high doses (i.e., at much higher doses than recommended). Specifically, it is an antagonist of the muscarinic acetylcholine receptors M₁ through M₅. These sedative and deliriant effects have in some cases led to the drug being used recreationally. Doxylamine was first described in 1948 or 1949.