The 1994 Dietary Supplement Health and Education Act (DSHEA) significantly curtailed the ability of the FDA to regulate supplements (sponsored by senators Harkin and Hatch, who coincidentally had the most campaign contributions from the supplement industry).
Still, the FDA can check to see if supplements are adulterated with actual drugs, which they can regulate. Between 2004 and 2012 the FDA recalled 237 supplements because they contained unlisted drugs in their ingredients. The most common type of supplement to be adulterated with drugs is for sexual function, followed by body building and then weight loss.
In a follow up study the FDA looked at the previously banned supplements that were still on the market. They found that 66.7% of those tested still contained unlisted drugs in their ingredients. Further, 22.2% of the supplements tested contained additional drug adulteration not previously detected.
These numbers are likely to be an underestimate because the FDA only tested for known and probable drug adulterants. For example they only tested the weight loss supplements for drugs typically found in weight loss supplements. They may also be missing newly discovered drugs or drug analogs.
There are other problems with the supplement market as well. A 2013 study using DNA barcoding on herbal supplements found that 59% contained ingredients not on the label. Only 48% actually contained the plant product they were supposed to, and a third of those also contained unlisted fillers and contaminants.
Other research has found that 20% of Ayurvedic herbal products were contaminated with unsafe levels of heavy metals.
Essentially, the supplement industry is poorly regulated, with frequent contamination, product substitution, and adulteration with unlisted drugs. Even after an FDA recall, 2/3 of previously adulterated supplements continue to spike their products with drugs. These pose a health risk to consumers who may have allergies, contraindications, or drug-drug interactions from the unlisted drugs in their supplements.