there is no evidence to suggest that they cause kidney failure that i know. all they do is speed up your metabolism and increase your body tempature so your body burns more fat. don't take them late in the day or they will keep you up late since they are loaded with caffine and ephedra, as long as you have a healthy heart you should be fine with all the ephedra fat burning supplements. ps i would reccomend Xenadrine, i'm on it now and it gives me a nice boost of energy along with the added fat burn.