RESEARCH MONOGRAPH · KDC-MN-377

D-Cycloserine

May 9, 2026 Kodiac biolabs Research Revised May 30, 2026 3 min read

Our opinion on this compound

70/100

Old TB antibiotic with a NMDA-partial-agonist side gig that's quietly become a fear-extinction workhorse

D-cycloserine is a strange compound. It was developed as a second-line tuberculosis antibiotic in the 1950s, a cyclic D-amino acid that inhibits alanine racemase and D-alanine ligase in the mycobacterial cell wall. Still on the WHO essential medicines list for MDR-TB. That's day job one.

Day job two is what got psychiatry interested: at low doses, far below the antibacterial range, DCS is a partial agonist at the glycine-B site of the NMDA receptor. About 50 percent intrinsic activity. That means at low NMDA tone it acts like an agonist, and at high NMDA tone it actually antagonizes. Which is why the dose-response in fear extinction studies is so finicky.

Here's the interesting part. Davis and Ressler at Emory pioneered the use of DCS as an augmenter of exposure therapy for phobias, PTSD, and OCD. The hypothesis: NMDA-dependent extinction learning gets stronger if you boost co-agonist occupancy during the learning window. Single doses, 50 mg, given 30 to 60 min before exposure sessions. The literature is genuinely mixed. Some trials positive, some null, some with bizarre dose-timing effects where chronic dosing actually impairs extinction (probably from receptor desensitization). The Mataix-Cols meta-analyses show a small positive effect that depends heavily on protocol.

What we like: as a research tool to probe NMDA partial agonism in vivo, DCS is the cleanest option. It crosses the blood-brain barrier, has a predictable PK, and the receptor pharmacology is well characterized.

What we don't like: the clinical translation has been frustrating. Negative trials in schizophrenia, mixed in autism, unclear in alcohol use disorder. Probably because the inverted-U dose-response is real and most trials never pinned down the right window for the right patient.

Sourcing is fine, DCS is an off-patent generic, available pharmaceutical grade, just check for diastereomeric purity since the L-isomer is the antibacterial one and not what you want for receptor work. Stability is the bigger issue, it racemizes in aqueous solution at neutral pH, so make solutions fresh.

Evidence: 70
Mechanism: 80
Reliability: 58
Safety: 70
Sourcing: 80
Value: 75
Signal: 62
Staying power: 68

Plain-language summary Intrigue 70 / 100

D-cycloserine is an old tuberculosis antibiotic isolated from a Streptomyces bacterium and approved in 1956. Decades later researchers noticed it also acts as a partial agonist at the NMDA glycine site, meaning it activates the receptor but only about halfway. That partial behavior makes its effect dose-dependent: at low doses it boosts NMDA function (because endogenous co-agonists are not saturating the site), at high doses it actually competes with the full natural agonists and dampens NMDA signaling. The low-dose facilitating effect inspired its use as an extinction-learning enhancer paired with exposure therapy for phobia, PTSD, OCD, and social anxiety. Trials show modest effects with strict timing requirements (give it shortly before therapy or it does not work). Canonical NMDA glycine-site partial agonist. 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.

NMDA receptor glycine-site partial agonist

An antibiotic with NMDA receptor partial agonist activity; investigated as an extinction-learning enhancer in exposure therapy for anxiety disorders.

Abstract

D-cycloserine ((4R)-4-amino-1,2-oxazolidin-3-one; CAS 68-41-7; molecular formula C3H6N2O2; molecular weight 102.09) is a cyclic amino acid antibiotic isolated from Streptomyces orchidaceus and approved by the FDA in 1956 for second-line tuberculosis treatment. The compound is a partial agonist at the NMDA receptor glycine site (intrinsic activity approximately 40 to 60 percent), and the partial agonism produces dose-dependent NMDA receptor modulation: at low concentrations relative to endogenous glycine/D-serine, D-cycloserine functions as a positive modulator (enhancing NMDA function); at higher concentrations it competes with the full-agonist endogenous co-agonists and behaves as a functional antagonist. The lower-dose facilitating effect underlies use as an extinction-learning enhancer in exposure therapy for phobia, PTSD, OCD, and social anxiety; clinical trials have shown modest effects with timing-dependent results (administration shortly before therapy session is critical). Plasma half-life is approximately 10 hours. Used as the canonical NMDA glycine-site partial agonist in research.

Mechanism of action

NMDA receptor glycine-site partial agonist (intrinsic activity 40 to 60 percent); facilitates NMDA function at low doses, antagonizes at high doses.

Reported research dose ranges

Antibiotic 250 to 500 mg in the published literature; extinction learning 50 to 250 mg before therapy session.

References

Ressler KJ, et al. Cognitive enhancers as adjuncts to psychotherapy: use of D-cycloserine in phobic individuals to facilitate extinction of fear. Arch Gen Psychiatry 2004.
Hofmann SG, et al. D-Cycloserine augmentation of cognitive behavioral therapy for anxiety disorders. JAMA Psychiatry 2013.
Mataix-Cols D, et al. D-Cycloserine augmentation of exposure-based cognitive behavior therapy for anxiety, obsessive-compulsive, and posttraumatic stress disorders: a systematic review and meta-analysis. JAMA Psychiatry 2017.

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KDC-MN-377

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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D-Cycloserine

Abstract

Mechanism of action

Reported research dose ranges

References

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Abstract

Mechanism of action

Reported research dose ranges

References

Read the full monograph

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