KDC-MN-152 – Kodiac biolabs

Plain-language summaryIntrigue 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 Kodiac editorial, not peer-reviewed.

Our opinion on this compoundEstablished workhorse

41/100

A 1969 TCA repurposed as a sleep drug; solid approval story, minimal research frontier appeal

Look, doxepin is a 1969 tricyclic antidepressant. The fact that someone noticed, decades later, that its H1 antagonism dominates at 3 to 6 mg and reformulated it as Silenor in 2010 is genuinely clever pharmacology. But clever is not the same as novel, and that distinction matters a lot for how we score this.

The mechanism insight here is dose-dependent receptor selectivity: at antidepressant doses (75 to 300 mg), doxepin hits muscarinic, alpha-1 adrenergic, and serotonergic receptors in ways that produce the classic TCA side-effect burden. Drop to the 3 to 6 mg range and what you’re left with is fairly selective H1 antagonism. That H1 blockade produces sleep continuity effects, meaning it targets maintenance insomnia, the 3am-wakeup phenotype, rather than onset. Thats a real and underappreciated clinical distinction. The plasma half-life of 15 hours is actually what makes it work; the drug is still present when early-morning sleep tends to fragment.

Here’s the thing, though: this story is 15 years old now. The Silenor approval trials (Krystal et al., Scharf et al., the 2010 package of phase 3 data) are well-replicated and well-designed. The evidence is solid. But the research conversation around sleep maintenance has largely moved on to orexin receptor antagonists. Suvorexant and lemborexant target the same clinical phenotype through a mechanism that most sleep researchers find considerably more interesting, and the dual-orexin-receptor angle has generated far more bench-level investigation over the last decade.

As a research probe, doxepin low-dose doesnt offer much that you couldnt get from a more selective H1 antagonist if H1 biology is what you’re actually studying. The 15-hour half-life makes washout kinetics awkward. Its not scheduled, which is a minor sourcing convenience relative to z-drugs or benzodiazepines, but that attribute alone doesnt make it an interesting bench target.

We think the honest placement here is: doxepin low-dose has good clinical evidence and a real approved indication, and it works. But “works for its indication” and “worth a serious investigator’s time in 2025” are different questions. The H1-for-sleep-maintenance story is mature and well-understood. The mechanism isnt revealing new biology. The differentiation from orexin antagonists cuts against it, not for it. If you’re building a sleep research program, this compound answers questions you mostly already know the answer to.

Novelty: 22
Mechanism promise: 25
In-category differentiation: 38
Evidence quality: 70
Translational signal: 68
Research bench value: 28

Rationale Doxepin low-dose scores as established-mature because the clinical evidence is real and replicated (FDA approval, multiple phase 3 RCTs for Silenor), but the mechanism is H1 antagonism, the literature is 15 years old on the low-dose reformulation angle, and orexin antagonists have largely superseded it as the more mechanistically interesting sleep-maintenance target. Low bench value as a probe compound drags the overall further.

Tricyclic antidepressant / H1 antagonist

A tricyclic antidepressant FDA-approved at low doses (3 to 6 mg) as Silenor for sleep maintenance insomnia, 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 approved at antidepressant doses (75 to 300 mg) since 1969 and reformulated at low doses (3 to 6 mg) by Somaxon and approved as Silenor in 2010 for sleep maintenance insomnia. At low doses, the compound shows selective H1 antihistamine activity; muscarinic, alpha-1 adrenergic, and serotonergic activities relevant at higher antidepressant doses 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.

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The full reference document covers compound identification, discovery and developmental history, mechanism of action, pharmacokinetics, sourcing and quality verification, and a curated reference list. Embedded inline below; download for offline reading.

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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.

Tag: KDC-MN-152

Doxepin (Low-Dose)

Abstract

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