Effects of the currently marketed form of loperamide (Imodium capsules) that might relate to abuse potential were examined. Study I was a double-blind "dose run-up" in adult male subjects with a history of illicit drug use but no history of opioid addiction. Subjective responses to doses of loperamide ranging from 12 to 60 mg were compared with responses to 120 mg codeine sulfate (96 mg base) and to placebo. Based on study I, loperamide (60 mg) was used in study II and its effects were compared with those of codeine (96 mg base) and placebo in an exaddict subject group. Study II subjects had had extensive opioid experience but were not actively addicted at the time of this double-blind, inpatient study. In study II, as in study I, unlike loperamide and placebo, codeine induced pupillary constriction. Loperamide (60 mg) induced a detectable subjective effect in somewhat over half the subjects, was "liked" little or not at all, and was identified as "dope" at a frequency less than that for a threshold dose of oral codeine. It was concluded that in its present form, i.e., capsules containing loperamide mixed with magnesium stearate, loperamide poses little threat of potential abuse.