a drug known to inhibit the effect of morphine and similar opiates. naloxone was used in the study of endorphins to demonstrate similarity to opiates. placebos administered to patients who believed they were taking the pain killer showed no effect when naloxone was given in addition. under normal conditions, placebos have been shown to lessen pain. an interesting experiment would be to administer naloxone to a runner to show the effect of this drug on a runner's high.