When you're doing a regular drug trial, you control using a placebo to account for the effect of just taking something. In this reverse analysis you need the o

When you're doing a regular drug trial, you control using a placebo to account for the effect of just taking something. In this reverse analysis you need the opposite: if the placebo effect is changing, that could either be because the placebo itself is more effective, or because people are reporting pain differently, or a number of other confounding factors. Analysing the people who took real painkillers too gives you a baseline to compare against.