Placebos are often overlooked in medical studies.

They are designed to be non-functional fake medicine, given to the control group in a blind trial. Placebos don’t have the active ingredients being tested in the trial, so their only impact should be psychological. They are often assumed simply to be “sugar pills.”

This got six researchers from the University of California, San Diego, McMaster University and the University of Oxford curious. They set out to analyze descriptions of placebo composition in 167 studies from four major journals from 2008 to 2009.

What they found, published in this week’s Annals of Internal Medicine, is that only about 8 percent of studies published in those top medical journals revealed what was in their placebos.

How do we know the placebo did not have an effect if we don’t know what the placebo was?

Indeed, the researchers highlight two cases in which ingredients were revealed and may have affected the study. In one older study of heart disease, control group patients were given placebos of corn oil or olive oil. We now know both can lower cholesterol levels. Another earlier study of cancer and HIV patients dispensed lactose sugar placebos, a problem when patients suffering from those diseases often suffer an increased risk of lactose intolerance.

So why the discrepancy and lack of transparency? Turns out the FDA doesn’t regulate what goes into placebos.

Next time you notice a reference to the often ignored placebo pill, check to see if the study describes the ingredients. You never know what’s inside.