Belief in theories

Don’t assume a plausible explanation is enough.

Just because we think we know how or why a health action might work, it doesn’t mean it will actually work or be safe.


Health actions that should work in theory often do not work in practice, or may turn out to be harmful. A plausible explanation of how or why a treatment might work does not prove that it actually does work, or that it is safe. Even if there is plausible evidence that a health action has benefits, the size of the benefits, and the health action’s safety, cannot be predicted without a fair comparison.


It used to be common to treat many different problems by cutting someone to make them bleed (bloodletting). People believed bloodletting worked by ridding the body of “bad humours” that they thought made people sick. But bloodletting did not help. It even killed people, including George Washington, the first president of the United States. His doctors drained 40% of his blood to treat a sore throat!

A more recent theory was that operating on blocked tubes (arteries) that carry blood to the brain would stop damage to the brain (strokes). That makes sense, but when that theory was tested in a fair comparison, researchers found not only that it did not help, but that some people died from the surgery.

Remember: Even when something people do for their health (a health action) seems to make sense, a fair comparison is still needed to get more reliable information about its effects.

Educational resources for this concept

Video: Does cough medicine really work? This is a Reactions video lasting 3:55 minutes.

Video: Does homeopathy work? This is a Reactions video lasting 4:31 minutes.

Video: Does melatonin do anything? This is a PBS video lasting 4:52 minutes.

Video: Can chocolate help you build muscle? This is a PictureFit video lasting 3:19 minutes.

Video: Should you really ‘feed a cold, starve a fever’? This is a SciShow video lasting 4:47 minutes.

Video: Why do some doctors still use bloodletting? This is a SciShow video lasting 5:56 minutes.

Video: Are egg yolks bad for you? – What you’ve heard might not be true This is a PictureFit video lasting 3:16 minutes.

Blog: Explanations about how treatments work can be wrong. Students 4 Best Evidence.

Essay: Why treatment uncertainties should be addressed. The James Lind Library.

Book: Here’s how it works. In Smart Health Choices.

Book: Advice on babies’ sleeping position, Drugs to correct heart rhythm abnormalities in patients having a heart attack, and Evening primrose oil for eczema. In Testing Treatments.

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