Question: How effective is the placebo effect?

Craig O'Hare answered on 10 Mar 2017:

This one is a bit hard to answer as I only know a few anecdotal cases where the placebo effect has shown to be effective. While at uni, I remember being shown a study showing that placebos could even activate a lot of immune related genes to on some level but I’ve not been able to find it since to see how believable it actually is.

While it’s not directly related to your question, you might like to know that mental state has a significant influence on the immune system. The field of psycho-neuroimmunology in particular looks at what stress does to the immune system. In a lot of cases stress can suppress the immune system but stir up auto-immune diseases like rheumatoid arthritis and lupus.