Nocebo is the opposite of Placebo

A placebo is a medicine that has no active ingredients, but which you believe to be effective, and your body either heals itself or (according to a controversial alternative theory) you merely tell the researcher that your body is healing in an unconscious effort to please the researcher. I hope I have described a placebo correctly.

A nocebo is the opposite.

Dr. Dean Edell - 'Nocebo' The Power Of The Mind Can Make You Sick

I should add that I believe Erin Brockovich worsened the nocebo effect on the people in the town her hit movie was about.