When Do Placebos Work And Why?

Placebos are particularly effective with conditions related to nervous system chemistry and immune response.

Do placebos and the power of the mind work? What I’ve found is yes, but not with everything. There are rules and conditions in which healing can be incredibly effective. Parkinson’s, chronic pain, irritable bowel syndrome, depression, anxiety, certain types of asthma, and autoimmune deficiencies are all very placebo-responsive. 

But cancer is not. . . .

Placebos have been particularly effective in the treatment of Parkinson’s disease. How do you explain that?

Parkinson’s is the perfect disease to talk about placebos. It is a chronic deficiency of dopamine, which is one of those brain chemicals that does a lot of jobs in our bodies. One of [dopamine’s] important roles is in reward processing: how we think about good things we might get in the future.

Expectation drives placebos. And dopamine is a chemical that’s very responsive to our expectations. Parkinson’s happens to be a deficiency in the very chemical that’s very important in placebo effects and rewards.

If you look at Alzheimer’s, which does not have a high placebo response, you start to see that there are rules at play when it comes to placebos. It’s not your brain magically doing all these crazy things. There are certain chemicals we have access to and others we don’t.

From a National Geographic interview with Erik Vance, the author of Suggestible You: The Curious Science of Your Brain’s Ability to Deceive, Transform, and Heal, which was published by National Geographic.

Some of this appears to be related to neuroendocrine systems in the body such as the HPA axis:

The hypothalamic–pituitary–adrenal axis (HPA axis or HTPA axis) is a complex set of direct influences and feedback interactions among three endocrine glands: the hypothalamus, the pituitary gland (a pea-shaped structure located below the hypothalamus), and the adrenal (also called "suprarenal") glands (small, conical organs on top of the kidneys). 

These organs and their interactions constitute the HPA axis, a major neuroendocrine system that controls reactions to stress and regulates many body processes, including digestion, the immune system, mood and emotions, sexuality, and energy storage and expenditure. It is the common mechanism for interactions among glands, hormones, and parts of the midbrain that mediate the general adaptation syndrome (GAS). While steroid hormones are produced mainly in vertebrates, the physiological role of the HPA axis and corticosteroids in stress response is so fundamental that analogous systems can be found in invertebrates and monocellular organisms as well. 

The HPA axis, HPG axis, HPT axis, and the hypothalamic–neurohypophyseal system are the four major neuroendocrine systems through which the hypothalamus and pituitary direct neuroendocrine function.

Conditions that do not relate to the neuroendocrine systems, nervous system chemistry or the immune system, in contrast, are not very responsive to placebo effects. They don't work, for example, to stop cancer, retard Alzheimer's disease, prevent blood loss, or to set broken bones. In other words, if your brain doesn't have control over any of the relevant bodily symptoms, placebos don't work.