An analysis of brain-boosting supplements finds they may contain multiple unapproved drugs, in combinations and doses that are potentially dangerous, according to NBC News.

“We don’t know how these drugs will affect human health,” said the study’s lead author, Dr. Pieter Cohen of the Harvard School of Medicine. “In some cases, there is a mixture of drugs combined in the same product that have never been tested together.”

Unlike pharmaceuticals, supplements are not strictly regulated by the U.S. Food and Drug Administration, the article notes.

The study, published in Neurology Clinical Practice, focused on compounds approved in other countries, but not in the United States. The doses on some of the labels were three times higher than what is recommended in those countries. In some cases, testing showed the doses were even higher than what was listed on the label, Dr. Cohen said.