MSD

Ephedra supplements (which contain ephedrine) are now illegal in the US because people couldn't figure out how to use them responsibly, and several died after using them.

Once ingested, ephedrine is metabolized into adrenaline, boosting the power of the matabolism, and also increasing heart rate and blood pressure. Taking ephedra supplements before exercising can lead to dehdyration and dangerous blood pressure levels. So, if you take it, then work out, or go running, or go to football practice in 95°+ weather, you're going to get overheated, dehydrated, and possibly have a heart attack. When used properly and responsibly, you should be fine, but it's too late to try that now.