Pain Reducing Effect of Opioids is Reliable

Is the Pain Reducing Effect of Opioid Medication Reliable? A Psychophysical Study of Morphine and Pentazocine Analgesia – Pain. 2013 Mar – Free full-text PMC article

A number of laboratory studies have confirmed the efficacy of opioid medication in reducing pain generated by a number of psychophysical modalities

However, one implicit assumption of clinical and experimental pain testing of analgesics is that the analgesic response is stable and will be comparable across repeated administrations

In the current study, the repeatability of opioid analgesia was assessed in a randomized, double-blinded study,  

  • using three psychophysical pain modalities (e.g., thermal, pressure, and ischemic)
  • over four medication sessions (two with active drug, two with placebo)

To determine the ability of a drug to reduce pain, four analytic methods (i.e., absolute change, percent change, ratio, and residualized change scores) were calculated to generate separate analgesic index scores for each measure and drug condition

Ischemic pain outcomes were moderately stable across sessions for both opioid medications;

however, heat and pressure analgesic index scores were moderately stable for only morphine and pentazocine, respectively

Finally, within stimulus modalities analgesic index scores were highly correlated with each other

In addition, analgesic responses to morphine and pentazocine demonstrate at least moderate reliability.

Implications of drug repeatability in genetic studies of analgesia


First, this study involved experimental pain testing in a young healthy population; therefore, the results may not be applicable to clinical pain

Several limitations of our study should be considered when interpreting the findings.

Only one dose of each medication was used, and we cannot comment on dose response relationships or the effects of repeated administration, as is often used in clinical settings.

Finally, the two drug sessions among women occurred in different menstrual cycle phases, and we recently reported that morphine responses tested against ischemic pain were greater in the follicular versus the luteal phase


The current study tested the repeatability of two opioid analgesics (morphine, pentazocine) across three commonly used psychophysical pain modalities

Drug efficacy was also compared among analytic methods, which demonstrated consistent analgesic responses on the three pain modalities following both medication

In general, reliability of analgesia between the first and second medication sessions was moderate, which differed depending on the modality (i.e., greatest with ischemic).   



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