The "nocebo" effect is classified as when a negative expectation results in a negative outcome, whether it's to do with movement, nutrition, a clinical treatment, or virtually anything else. Truly, it's the other far-side of the spectrum from the placebo.
This ties directly in with my past topics about how language selection and expectations of pain can propagate and cause more pain. Like I've said before, mental state has a huge effect on driving the body's outputs, whether it be pain, inflammation, or more.
This being said, the nocebo has a massive influence on the effectiveness of a therapy treatment. If a client does not believe in a certain type of treatment or modality or they think that it will cause them harm, then it absolutely can interfere with the treatment outcome!
For this reason, I do my best to thoroughly educate my clients in exactly what kind of processes are occurring in their body before, during, and after any treatment I provide in order to help them understand the targeted effect and increase the probability of success.
If a client simply doesn't believe in a modality at all, then that's ok! Therapists work for the client, not the other way around, and the client does not HAVE to subscribe to every philosophy. However, in absence of that belief, gears need to be switched with either a new approach or a new therapist, otherwise we're charging that client continually for treatments with no outcome. And that's where responsibility on the clinician's side comes in.
The brain is a funny thing, but it's the boss. We gotta work with it.