The aim of the paper is to define the main terms from the field of the research on placebo and their relationships. Included are placebo, the placebo effect, the opposite placebo effect, the nocebo effect, the placebo side effect and the nocebo side effect. Presented is the integration of these terms into the coherent system of possible effects of placebo. In order to determine which effect occurs suggested are three objective and observable bipolar criteria: the outcome (positive vs. negative), the place in other effects of placebo (main vs. side), and the relationship between the effect of placebo and the action of the active therapy (consistent vs. not consistent with the action of the active therapy). Proposed system predicts eight possible effects of placebo. Five of them correspond to the main terms from the field of the research on placebo. The remaining three are new and require further research.