Double blinding, where both the researcher and the participant don’t know if they have received the active agent, can be a common feature in many (but not all) clinical trials.  In some instances, the fact the participant has received the experimental agent can be self-evident, but not always in a way that’s quite as funny as this Don Mayne cartoon.  What this cartoon does raise is that there should be a discussion with potential participants of the impact of the tested agent working ‘very well’ and whether this could have deleterious impacts.