The official policy of the American Medical Association:
“Use of a placebo without the patient's knowledge may undermine trust, compromise the patient-physician relationship, and result in medical harm to the patient.
A placebo must not be given merely to appease a difficult patient, because doing so serves the convenience of the physician more than it promotes the patient's welfare.
Physicians may use placebos for diagnosis or treatment only if the patient is informed of and agrees to its use.

Placebos won't lower your cholesterol or shrink a tumor. Instead, placebos work on symptoms modulated by the brain, like the perception of pain. They have been shown to be effective for things like pain management, stress-related insomnia, and cancer treatment side effects like fatigue and nausea.

One of the most common theories is that the placebo effect is due to a person's expectations.

All communication can have a beneficial or harmful effect. One study found that teaching communication skills to families reduced anxiety and depression. On the other hand, couples who dwell on problems and negative aspects of their relationships were shown in a study to have weaker immune systems.