It has been shown that even if you tell a patient this pill is placebo and it's not going to help you, there's still some positive effect. It's all about perception. There's a really good lecture about it here:

https://www.youtube.com/watch?v=LWQfe__fNbs

Is there scientific proof we can heal ourselves? Lissa Rankin, MD at TEDxAmericanRiviera

It has been shown that even if you tell a patient this pill is placebo and it's not going to help you, there's still some positive effect. It's all about perception.

There was a really good TED talk with the topic "Is there scientific proof we can heal ourselves?" at TEDxAmericanRiviera presented by Lissa Rankin, MD.