I'm willing to bet oxytocin would work. One effect is to make people more trusting, so if you coupled that with an authority figure in a white lab coat intoning

I'm willing to bet oxytocin would work. One effect is to make people more trusting, so if you coupled that with an authority figure in a white lab coat intoning "Take this pill, and you'll get better," it would probably have a high placebo effect rate.