Are dietary supplements risky?

This is how gssi.com answers this question
Dietary supplements-unlike medicines and other drugs-do not undergo rigorous testing and screening for efficacy and safety.
Some products sold as dietary supplements, especially those containing ephedrine, are associated with serious-and sometimes deadly-adverse side effects. Additional risky supplements include androstenedione and other "prohormone" precursors to testosterone, yohimbine, and products that contain kava.
Even supplements like vitamins, caffeine, creatine, and protein powders that are safe when taken in recommended doses could be harmful if taken in large doses for a long time.
Dietary supplements may contain substances not shown on the package label that may be harmful or can lead to a positive doping test in sport competitions.
How do you know if a product is safe? You don?t. However, vitamin and mineral supplements labeled USP (United States Pharmacopoeia) have passed tests for dissolution, disintegration, potency, and purity. In addition, nationally known food and drug manufacturers generally make supplements under the strict quality control procedures they already have in place.