The ethical aspects of placebo control in clinical trials have been extensively and controversially debated in the last decade. However, a thorough analytical comparison of the different existing international regulations, their terminologies and their ethical principles concerning placebo, is still missing. The central issue in the ongoing controversy is the justification of placebo-use, if proven treatment exists. All present versions of the examined guidelines propose such justifications, but each guideline differs from the others in relevant details. Therefore the conditions justifying placebo-use according to each guideline are the focus of our attention. We will first propose a formalized general principle that defines the ethical acceptability of placebo-use. Then we will analyse three categories of conditions put forward by the different documents: the risk of harm or burden, compelling scientific reasons, and the availability of proven treatment. The analysis shows important normative discrepancies and contradictions between the examined guidelines. Especially striking is the fact that some guidelines allow the participants in clinical trials to be exposed to a risk of serious harm, while others do not. Finally, we try to show how the normative difference of each guideline could influence the decision of researchers or IRBs concerning the ethical acceptability of placebo-use.