Fig. 3.

Expectation-induced placebo analgesia and its blockade by naloxone. A, A hidden injection of naloxone on days 2 and 4 (group 2) does not produce any change in pain tolerance compared with days 1 and 3, indicating that naloxone per se does not affect this type of experimental pain. B, An open injection of saline (group 3) produces a placebo analgesic effect. Days 1 and 3 represent preinjection and postinjection controls.C, An open injection of naloxone on day 2 (group 4) blocks the placebo effect completely. In fact, pain tolerance on day 2 is equal to preinjection and postinjection controls.