Ephedra
This ephedra stuff is scary. WP article notes that:
Ephedra has been linked to lifethreatening side effects, even when used by outwardly healthy people at recommended doses, because it speeds heart rate and constricts blood vessels. Those effects can be exacerbated by exercise and use of other stimulants such as caffeine...
I'm no bodybuilder, but a couple years ago I saw the ads for ephedra products in Men's Health (think Ripped Fuel), and considered using them. I chose not to, but only after doing sufficient research on the Internet to find that the potential side effects were frightening.
As long as the FDA is in the business of regulating drugs, they ought to be regulating these supplements. I think their inactivity sends the wrong message, as it implies to consumers that all herbal supplements are more safe and harmless than the medicines and drugs that do fall under FDA regulation. This dichotomy is itself dangerous.
UPDATE: CNN/SI's Alexander Wolff lays out a good case for regulating ephedra, but Tim Layden makes some interesting points on why regulation may have limited effects (at least for athletes).
