Doxylamine

Doxylamine is one of the many sedating antihistamines used by itself as a short-term sedative, and in combination with other drugs as a night-time cold and allergy relief drug. It is also used in combination with the analgesics paracetamol (acetaminophen) and codeine as an analgesic/calmative preparation, and is prescribed in combination with vitamin B₆ (pyridoxine) to prevent morning sickness in pregnant women.

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 ^[2] and is more sedating than some prescription hypnotics. One study found that doxylamine succinate was more effective than the barbiturate phenobarbital for use as a sedative.^[2]

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. A recent placebo-controlled, double-blind randomized trial found the formulation of doxylamine and pyridoxine marketed as Diclectin to be effective in controlling nausea and vomiting due to pregnancy.^[3]

Metabolites

The two main metabolites are desmethyldoxylamine and didesmethyldoxylamine.

Side effects

Doxylamine succinate is a potent anticholinergic and has a side-effect profile common to such drugs, including dry mouth, ataxia, urinary retention, drowsiness, memory problems, inability to concentrate, hallucinations, psychosis, and a marked increased sensitivity to external stimuli. Like many hypnotics, it should not be combined with other antihistamines, such as Zyrtec (cetirizine) or diphenhydramine, as this combination can increase the risk of serious side effects. Using doxylamine over a long period of time is not recommended and can cause dependence. However, the drug is not addictive and withdrawal effects are unlikely to be experienced with prolonged use.

Toxicity

Doxylamine succinate is generally safe for administration to healthy adults. Typical preparations that contain doxylamine range from 6.25 mg to 50 mg. The LD₅₀ is estimated to be 50–500 mg/kg in humans.^[4] Symptoms of overdose may include dry mouth, dilated pupils, insomnia, night terrors, euphoria, hallucinations, seizures, rhabdomyolysis, and death.^[5] Fatalities have been reported from doxylamine overdose. These have been characterized by coma, tonic-clonic seizures and cardiorespiratory arrest. Children appear to be at a high risk for cardiorespiratory arrest. A toxic dose for children of more than 1.8 mg/kg has been reported. A 3 year old child died 18 hours after ingesting 1,000 mg doxylamine succinate.^[6] Rarely, an overdose results in rhabdomyolysis and acute renal failure.^[7]

Studies of doxylamine's carcinogenicity in mice and rats have produced positive results for both liver and thyroid cancer, especially in the mouse.^[8] The carcinogenicity of the drug in humans is not well studied, and the IARC lists the drug as "not classifiable as to its carcinogenicity to humans".^[9]

Formulations

Doxylamine is primarily used as the succinic acid salt, doxylamine succinate.

• It is the sedating ingredient of NyQuil.
• In Commonwealth countries, such as Australia, South Africa and the United Kingdom, doxylamine is available prepared with paracetamol (acetaminophen) and codeine under the brand name Dolased, Propain Plus, 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, Dormidina, Restavit, Unisom-2 and Sleep Aid (generic - Australia),.
• In the United States doxylamine succinate is the active ingredient in the over-the-counter sleep-aid tablets branded as Unisom; however, the gel-cap form contains diphenhydramine hydrochloride instead.^[10]
• In Canada doxylamine succinate is the active ingredient in the over-the-counter sleep-aid tablets branded as Unisom 2; while Unisom contains diphenhydramine hydrochloride as the active ingredient.
• 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.

References