If the placebo effect means you feel better just because you believe a certain treatment is going to work, then the nocebo effect means that you feel worse just because you expect the worst.
In treating patients with phobias, therapists address the emotion of fear and try to alter that emotion so the patient is no longer afraid. However, patients with what doctors call blood-injection-injury phobia experience more difficulty with treatment. It includes people who fear needles, blood, and surgery.