[Placebo response: in studies on pain and under other clinical conditions].

This contribution compares unexplained essential questions regarding the placebo response with current empirical evidence: (1) Are the placebo response rates equivalent in the groups treated with medication or placebo? Very little evidence has been gathered to support this generally accepted additivity while some findings negate its validity. (2) Is the placebo response a function of the probability of receiving medication or placebo? There are indications that the number of study groups included in a trial determines the level of placebo and medication response. (3) How great is the placebo response in trials that directly compare a (new) medication with one that for example is already on the market? There are indications that such comparative studies produce higher placebo response rates. (4) How high is the placebo response rate in everyday clinical practice--or does the response to a medication in trials substantiate the effect of the medication in everyday clinical practice?