RESEARCH MONOGRAPH · KDC-MN-152

Doxepin (Low-Dose)

May 9, 2026 Kodiac biolabs Research Revised May 30, 2026 2 min read

Our opinion on this compound

73/100

An old tricyclic at homeopathic-by-TCA-standards doses that turns out to be a quietly effective sleep tool

Low-dose doxepin (3 to 6 mg) is one of those reformulations where the same molecule at very different doses produces fundamentally different pharmacology. Standard antidepressant doxepin doses (75 to 300 mg) have substantial activity at SERT, NET, muscarinic receptors, alpha1 adrenergic, and H1. At 3 to 6 mg the H1 antihistaminergic activity dominates almost entirely, because doxepin has roughly 100-fold higher affinity at H1 than at most of the other targets, so you're titrating into a window where H1 occupancy is meaningful but the other receptor effects are essentially negligible.

Silenor (the branded low-dose formulation, FDA-approved 2010) trial data showed efficacy in sleep maintenance insomnia particularly, with sleep onset effects more modest. The mechanism is histaminergic blockade in the tuberomammillary nucleus and downstream, which is one of the cleanest sleep-relevant pharmacologic actions available.

What we like: this is one of the few sleep agents with essentially zero abuse liability and zero direct effect on GABA-A. The next-morning effects are surprisingly minimal given doxepin's reputation as a sedating tricyclic; at these low doses the daytime carryover is much less than youd expect because the H1 occupancy curve is fairly steep. As a research tool for studying histaminergic contributions to sleep architecture its useful and underappreciated.

What we dont like: the dose-response is finicky. Going from 6 mg to 10 mg starts to bring in other receptor effects (cholinergic in particular), and the difference between an antihistaminergic sleep aid and a mildly anticholinergic tricyclic is small. Also there's some pharmacokinetic variability between people, with CYP2D6 metabolism affecting clearance. People who slow-metabolize can get more carryover than the average response would suggest.

Mechanism: H1 receptor antagonism in wake-promoting histaminergic neurons. Well-mapped at the receptor level, less so on the downstream sleep architecture effects.

Sourcing: long-established generic, manufactured at scale, research-grade quality is reliable. Just be careful with formulation if you're using research-grade powder; the 3 mg low-dose effect requires accurate weighing.

Slept-on sleep tool, no pun intended. The H1-selective angle at low dose is what makes it interesting and most marketing positions it next to other tricyclics rather than as the antihistamine it actually is at this dose range.

Evidence: 70
Mechanism: 80
Reliability: 70
Safety: 75
Sourcing: 85
Value: 75
Signal: 60
Staying power: 65

Plain-language summary Intrigue 60 / 100

Doxepin is an old tricyclic antidepressant. At very low doses (3-6 mg, sold as Silenor) it works as an H1 antihistamine sleep aid without the antidepressant or anticholinergic effects of full doses. Not stocked by Kodiac. This monograph is provided for research and educational reference.

Intrigue 0–100 blends mechanism novelty, evidence strength, and translational potential. Kodiac editorial, not peer-reviewed.

Tricyclic antidepressant / H1 antagonist

A tricyclic antidepressant studied at low reported research dose ranges (3 to 6 mg) for sleep maintenance, exploiting selective H1 antihistamine activity at the low end of the dose range.

Abstract

Doxepin (Silenor at low doses; CAS 1668-19-5) is a tricyclic antidepressant. At low reported research dose ranges, the compound shows selective H1 antihistamine activity; muscarinic, alpha-1 adrenergic, and serotonergic activities relevant at higher dose ranges are minimal. The H1-mediated sleep maintenance effect targets sleep continuity rather than sleep onset. Pharmacokinetics: plasma half-life 15 hours; CYP2D6 and CYP1A2 metabolism. Schedule status: prescription-only; not federally scheduled.

Mechanism of action

At low dose: selective H1 antihistamine. At antidepressant doses: tricyclic with serotonergic and adrenergic activity.

Reported research dose ranges

Reported research dose ranges in the literature span 3 to 6 mg.

References

Krystal AD, et al. Efficacy and safety of doxepin 1 mg, 3 mg, and 6 mg in adults with primary insomnia. Sleep 2011.

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KDC-MN-152

The full reference document is provided strictly for research use only. It reports research dose ranges from the published literature, not instructions for use in humans or animals.

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FOR RESEARCH USE ONLY. Not for medical, diagnostic, or therapeutic purposes. Not for human consumption. All information is provided for research and educational purposes only.

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Doxepin (Low-Dose)

Abstract

Mechanism of action

Reported research dose ranges

References

Read the full monograph

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Abstract

Mechanism of action

Reported research dose ranges

References

Read the full monograph

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