In this paper I shall discuss the placebo effect from a posthuman angle. The placebo effect is a medical conundrum, as it is a medical effect that is produced by “nothing”. Placebo literally means, ”I please”, and the placebo has, among other things, been defined as an inert substance, often a calcium pill. Placebos are being used in medical trials to determine how much of the medical effect is caused by other factors than medical. There is a vast amount of literature on the placebo effect and it has been studied since the late 1940’ies, mainly for the purpose of pre-elimination from medical trials. It has been studied as an effect of personality traits, as an expectational effect, and from a physiological point of departure. Still it remains a medical riddle how something that is “nothing” can cause a measurable effect? In this paper I shall address this issue from a posthuman angle, applying Karen Barad’s concept of agential realism to the problem. I argue that the placebo effect is a cuttingtogether- apart that produces specific agencies in the placebo phenomenon – that is, both the subject under treatment and the placebo emerge in the placebo effect. Through quantum physics, Barad proposes a reversal of cause and effect. In brief, before measurement the qualiry of light is indetermined, and thus can be both particle and wave, but after measuerement it can never be both. Even the very act of setting up an apparatus to determine which way a photon will pass, will “turn light into particles”. Thus, examining the placebo effect in these terms, I want to propose, that before the placebo effect, the placebo can be both inert and active, and the subject receiving the placebo can be both reactant and non-reactant – both are indetermined. But in the act of measuring the placeboeffect, the substance becomes active, albeit dependent on other factors, which are non-visible in the experimental setting.