Contaminants–some of them serious–were found in 90% of herbal supplements tested

The majority of dietary supplement facilities tested were found noncompliant with good manufacturing practices guidelines.  Some supplements were found to: contain heavy metals; contain far less or far more of the active ingredient than advertised; be adulterated with prescription drugs; be contaminated with dangerous mycotoxins (molds).

Most people are unaware that supplements no longer have to be approved by the government or that supplement ads don’t have to be vetted. See here for more detail.