Americans spend an estimated $5 billion a year on unproven herbal supplements that promise everything from fighting off colds to curbing hot flashes and boosting memory. But now there is a new reason for supplement buyers to beware: DNA tests show that many pills labeled as healing herbs are little more than powdered rice and weeds.

Using a test called DNA barcoding, a kind of genetic fingerprinting that has also been used to help uncover labeling fraud in the commercial seafood industry, Canadian researchers tested 44 bottles of popular supplements sold by 12 companies. They found that many were not what they claimed to be, and that pills labeled as popular herbs were often diluted — or replaced entirely — by cheap fillers like soybean, wheat and rice.