Although these drug-like effects of placebos represent an interesting phenomenon in the clinical setting, they also have implications that have been ignored so far. One of these has to do with the use of drugs in sport competitions to boost physical performance. Among performance-boosting drugs, morphine is known to be a powerful analgesic that increases tolerance to pain, thereby improving physical performance [World Anti-Doping Agency (WADA), www.wada-ama.org]. The importance of opioid-mediated placebo responses consists in the fact that they can be exploited when one wants morphine-like effects without giving morphine. For example, in the context of pain management, it has been shown that morphine administration for 2 d in a row may induce robust placebo analgesic responses when morphine is replaced with a placebo on the third day (Amanzio and Benedetti, 1999). This raises the important question whether two morphine administrations separated several days or weeks from each other have similar powerful effects on subsequent placebo responses.