Indications

Doxylamine is a member of the ethanolamine class of antihistamines and has anti-allergy power superior to almost every other antihistamine on the market, with the exception of diphenhydramine (Benadryl). It is also the most effective over-the-counter sedative available in the United States, and is more sedating than some prescription hypnotics. In a study, it was found that doxylamine succinate is possibly more effective than the barbiturate phenobarbital for use as a sedative. For this reason, doxylamine has sometimes been used off label in a manner similar to diphenhydramine for the reduction of anxiety symptoms.

The dosage required to induce hypnosis (sleep) can be as low as 6.25 mg, but is usually effective in dosages of up to 25 mg. Higher doses are not recommended by the United States Food and Drug Administration, although single dosage recommendations of up to 50 mg are common in some countries, including Australia, where it is marketed under the names Restavit and Dozile.

Metabolites

The 2 main metabolites are desmethyldoxylamine and didesmethyldoxylamine.

Formulations

It is the sedating ingredient of NyQuil.
In Commonwealth countries, such as Australia, Canada, and the United Kingdom, doxylamine is available prepared with paracetamol (acetaminophen) and codeine under the brand name Dolased , Syndol or Mersyndol, as treatment for tension headache and other types of pain.
Doxylamine succinate is used in general over-the-counter sleep-aids branded as Somnil, Dozile, Donormyl, Restavit and Unisom-2.
In the United States doxylamine succinate is the active ingredient in the over-the-counter sleep-aid tablets branded as Unisom; however, according to the manufacturer's website, the gel-cap form contains diphenhydramine hydrochloride instead.
It is also available in combination with vitamin B₆ and folic acid under the brand name Evanorm (marketed by Ion Healthcare).
In Canada, doxylamine succinate and pyridoxine (vitamin B6) are the ingredients of Diclectin, which is used to prevent morning sickness.

Side effects

Doxylamine succinate is a potent anticholinergic and has a side-effect profile common to such drugs, including dry mouth, ataxia, urinary retention, and drowsiness. Other drugs with well known anticholinergic effects include many antipsychotic and tricyclic antidepressant drugs.

Toxicity

Doxylamine succinate is generally safe for administration to healthy adults. Typical preparations that contain Doxylamine range between 6.25mg to 25.0mg. The LD50 is estimated to be between 50-500 mg/kg in humans. Symptoms of overdose may include dry mouth, dialated pupils, insomnia, euphoria, hallucinations, and seizures. Fatalities, while rare, have been recorded.