Meta-Analysis Finds Body's 'Natural Painkillers' Move the Needle Only Slightly

A pooled analysis of 67 trials that blocked opioid receptors in healthy volunteers found pain rose only a little, a result that complicates the textbook description of endogenous opioids as the body's main defense against everyday pain.

Blocking the body's own opioid receptors in healthy adults raises pain only slightly, according to a systematic review and meta-analysis published online in the journal PAIN on September 14, 2026. Researchers led by Isabell M. Meier and Siri Leknes at the University of Oslo pooled 665 effect sizes from 67 double-blind, placebo-controlled trials, covering 2,706 healthy volunteers, in which participants received a centrally acting opioid antagonist such as naloxone or naltrexone before standard pain testing. Averaged across pain intensity, unpleasantness, threshold and tolerance, blockade raised pain by a Hedges' g of 0.18 (95 percent confidence interval, 0.11 to 0.25), the equivalent of about 2.8 points on a 0 to 100 pain intensity scale.

The researchers, who searched four databases from October 2020 to June 2025 for their preregistered review, found that the effect grew somewhat larger at higher levels of receptor blockade, but statistical tests for reporting bias indicated the pooled estimate was itself probably inflated by a tendency for smaller studies with bigger effects to be published. Heterogeneity across the 67 studies was substantial, and differences in study design, such as which drug was used or where on the body pain was tested, explained little of that variation.

The finding bears on a widely cited textbook account in which naloxone is used to "reverse" pain relief and is taken as proof that endogenous opioids are doing the relieving, a logic that underpins much of the research literature on placebo analgesia and on why treatments such as exercise or acupuncture are popularly said to work by releasing the body's own opioids. The authors reported that the most credible evidence for endogenous opioid pain regulation in their dataset came specifically from the subset of studies testing whether an antagonist could reverse placebo-induced pain relief, rather than from studies of baseline, everyday pain sensitivity. They concluded that, outside that placebo-analgesia context, endogenous opioids play only a minor role in regulating pain in healthy people.

The authors did not test opioid medications themselves, which act on the same mu-opioid receptors but at doses far exceeding what the body produces on its own, and the review says nothing about whether opioid drugs relieve pain in patients. The paper's authors called for further work to clarify why the placebo-reversal effect looks more robust than the everyday pain-regulation effect, and whether that pattern holds in people with chronic pain rather than in healthy volunteers tested under laboratory conditions.