The Pill That Turned Ketamine Off: What Naltrexone Reveals About Mechanism

In 2018, a Stanford team ran a small trial that they did not finish. They gave adults with treatment-resistant depression ketamine, and gave some of them naltrexone first, an opioid-receptor blocker. At the interim analysis, they stopped the study because the pattern was already clear enough that continuing felt unethical. In the ketamine-plus-placebo group, most participants responded. In the ketamine-plus-naltrexone group, the antidepressant effect largely did not appear. The dissociation showed up in both. Only the mood benefit went missing.

That is a strange and important result, because it does not fit the story most of us were taught about how ketamine works.

Williams and colleagues (2018, American Journal of Psychiatry) pretreated patients with oral naltrexone, then gave ketamine, in a crossover design. Seven of twelve participants met response criteria in the ketamine-plus-placebo condition. Under naltrexone, the reductions in depression scores were significantly smaller. Crucially, naltrexone did not blunt the dissociative experience. It selectively interfered with the antidepressant response.

The investigators halted the study at the interim analysis on ethical grounds, since it appeared they were withholding benefit from the naltrexone arm. That decision is defensible; interim-stopped trials tend to overstate effect sizes, and the sample was tiny.

The clean version of ketamine’s story is that it is an NMDA-receptor antagonist and its antidepressant action follows from that. This finding complicates that story by implicating mu-opioid receptor activity in the mood effect. That is genuinely interesting, and it provoked a sharp back-and-forth in the literature about replication and interpretation.

This is not a demolition of the NMDA story. One small crossover trial, stopped early, in treatment-resistant depression rather than pain, is a signal, not a verdict. It does not make ketamine an opioid, and it does not mean ketamine works only through opioid receptors. It fits a broader pattern: ketamine’s mood and pain effects often run on partly separate tracks. I think anyone who recites the tidy NMDA-only mechanism to a patient should add the word "probably."

The 2018 trial used naltrexone at a full opioid-blocking dose. Low-dose naltrexone, the 1.5 to 4.5 mg regimen many chronic pain and autoimmune patients take, was not tested. Whether LDN blunts ketamine’s effect is not established. Treat it as an open question; ask about LDN specifically during intake; and coordinate timing with the prescriber rather than assuming it is either safe or blocking.

As a therapist or KAP provider, you are not managing the medication. You do need to know if it is on board. This is an intake question, not an afterthought.

A patient on a naltrexone injectable heading into ketamine work needs a prescriber conversation before the first session. This should never be a surprise discovered mid-series. Never advise a patient to stop naltrexone to 'make the ketamine work.' Stopping naltrexone can restore opioid sensitivity and raise overdose risk in someone with an opioid history. That timing decision is the prescribing clinician's.

In a small 2018 trial, full-dose naltrexone significantly reduced ketamine’s antidepressant effect in treatment-resistant depression while leaving dissociation intact, and the study was halted at interim. It suggests opioid-receptor involvement in ketamine’s mood effect. It was small, was stopped early, has not been cleanly replicated, and tested depression rather than pain, so the takeaway is a signal rather than a settled fact.

The 2018 trial used full-dose naltrexone, not LDN, so its findings do not directly answer this. Whether low-dose naltrexone blunts ketamine is not established. Treat it as an open question; ask about LDN specifically during intake; and coordinate timing with the prescriber rather than assuming it is either safe or blocking.

That is a prescriber’s decision, not a therapist’s, and it carries real risk. Stopping naltrexone can restore opioid sensitivity and raise overdose risk in someone with opioid use history. Never advise stopping a medication to improve a ketamine response. Flag the interaction, document it, and refer the question about timing to the prescribing clinician.

Peter H. Addy, PhD, LPC, LMHC, is a Portland-based licensed therapist and the founder of Psychedelic Affirming Education, an NBCC-approved continuing education provider for licensed mental health professionals and Oregon Psilocybin Services facilitators. His research background includes work at Yale School of Medicine on psychedelic substances. Learn more.

Ketamine’s opioid-system involvement is not a footnote when you are assessing misuse risk. Our course, Foundations of Ketamine Addiction (2.5 CE credits), covers the neurobiology, risk factors, and assessment within which this mechanistic question sits. If you work with ketamine, this is the piece that helps you ask better questions and catch risk earlier.