Doxylamine: English spelling of doxylamine (H1 antihistamine, sleep aid)

Doxylamine is the English spelling of the active ingredient doxylamine, an H1 antihistamine of the first generation from the class of ethanolamines. In English-speaking regions, this form is commonly encountered on imported packages, in international studies, and in obstetrics, where the combination preparation doxylamine pyridoxine (Diclectin in Canada, Bonjesta and Diclegis in the USA) has been used for decades against nausea and vomiting of pregnancy. Doxylamine is identical in content to doxylamin.

In Germany, doxylamine is available over the counter in pharmacies as a sleep aid (Hoggar Night, Schlafsterne, Mereprine) and as a component of cold and cough combination preparations (Wick MediNait). The typical indication is short-term treatment of sleep-onset insomnia in otherwise healthy adults and symptomatic treatment of nausea and vomiting of pregnancy.

Mechanism of action

Doxylamine is a competitive antagonist at the H1 histamine receptor. It crosses the blood-brain barrier and blocks H1 receptors in the histaminergic arousal system (tuberomammillary nucleus, TMN). Histamine is one of the central wake neurotransmitters; its blockade leads to sedation and drowsiness. This effect occurs rapidly, approximately 30 minutes after oral administration.

Doxylamine has additional strong anticholinergic properties (blockade of muscarinic receptors), which explains dry mouth, accommodation disorders, constipation, and urinary retention. The half-life is approximately ten hours, so doxylamine can cause morning-after hangover effects the next day, especially in elderly patients and those with slowed metabolism.

The antiemetic effect in nausea and vomiting of pregnancy is mediated via H1 and cholinergic receptors in the chemoreceptor trigger zone of the medulla oblongata. In combination with pyridoxine (vitamin B6), a synergistic effect occurs with better tolerability than with monotherapy.

Indications

• Acute sleep-onset insomnia in adults: short-term use for a few days
• Nausea and vomiting of pregnancy (hyperemesis gravidarum): in combination with pyridoxine vitamin B6, first-line therapy after dietary measures
• Symptomatic treatment of rhinitis: in cold and cough combination preparations (Wick MediNait)
• Kinetosis (motion sickness): off-label, replaced by dimenhydrinate

Dosage and administration

Sleep disturbance in adults: 25 to 50 mg approximately 30 to 60 minutes before bedtime. Maximum duration of use without medical consultation is two weeks to avoid dependence, tolerance, and rebound insomnia.

Nausea and vomiting of pregnancy: Doxylamine 10 mg plus pyridoxine 10 mg, usually two tablets at night and one each in the morning and afternoon depending on the severity of nausea. Adjustment based on individual symptom severity.

Renal insufficiency and hepatic insufficiency: Halve the dose and use with caution. Elderly patients: Doxylamine is on the PRISCUS list of potentially inappropriate medications in older adults due to pronounced anticholinergic effects; use should be avoided if possible or kept to the shortest duration.

Side effects

Frequent: Next-day drowsiness (hangover), dry mouth, constipation, accommodation disorder, dizziness, headache, concentration disturbances, paradoxical excitation in children and elderly.

Occasional: Tachycardia, urinary retention, confusion, falls especially in elderly, rash.

Rare: Acute angle-closure glaucoma, severe allergic reactions, photosensitivity, hematological changes.

Important: Doxylamine contributes significantly to anticholinergic burden and is associated with increased risk of cognitive decline and dementia with long-term use. Longer-term evening use is therefore viewed critically.

Interactions

• Other CNS depressants (alcohol, benzodiazepines, opioids, Z-substances): additive sedation with risk of respiratory depression
• Other anticholinergic agents (tricyclic antidepressants, tolterodine, atropine): additive anticholinergic effects
• MAO inhibitors (tranylcypromine): enhanced and prolonged anticholinergic effect
• Cholinesterase inhibitors (donepezil, galantamine): mutual cancellation of effects
• Serotonergic agents: theoretical risk of serotonin syndrome, clinically poorly documented

Special precautions

Pregnancy: After decades of use, doxylamine is considered safe in pregnancy, especially in combination with pyridoxine for nausea and vomiting of pregnancy. An observational study of more than 200,000 pregnancies (Canadian registry) found no increase in fetal malformations. Breast-feeding: Doxylamine enters breast milk in small amounts and can sedate the infant; short-term use is possible, long-term use is not recommended.

Not for long-term sleep treatment: Hoggar Night and similar products are intended only for short-term use. For chronic insomnia, diagnostic evaluation is indicated (sleep hygiene, cognitive behavioral therapy for insomnia CBT-I, polysomnography). Long-term use leads to tolerance and cognitive risks.

Ability to drive: Even the next morning, doxylamine can significantly reduce reaction ability. Driving or operating machinery should be avoided, especially in the first few days.

Mixed intoxications: Doxylamine overdoses are common in emergency departments, especially in suicide attempts. Symptoms resemble atropine-containing poisoning with tachycardia, mydriasis, confusion, seizures, and possible cardiac arrhythmias.

You might also be interested in

• Diphenhydramine, another H1 antihistamine as a sleep aid
• Dimenhydrinate, antiemetic antihistamine
• Melatonin, alternative for sleep disturbances
• Zolpidem, Z-substance for short-term insomnia
• Pyridoxin, vitamin B6 combination partner for nausea and vomiting of pregnancy

Frequently asked questions

Sources

• Gelbe Liste, Doxylamin active ingredient profile
• ACOG Practice Bulletin Nausea and Vomiting of Pregnancy
• PRISCUS List 2.0, potentially inappropriate medications in older adults
• BfArM, Federal Institute for Drugs and Medical Devices