It’s all in your head. That’s often the explanation when patients are fed sugar pills and suddenly recover, benefiting from the so-called placebo effect. Since they were conceived in the 18th century, placebos have most often been used as a control to measure actual drugs’ efficacy. But now researchers are finding they may actually have physical benefits, especially in patients with low levels of an enzyme called COMT. That could also help drugmakers to weed out test subjects who are prone to respond to sugar pills — thus minimizing effective drugs’ results.