Long story short, I've taken natural supplements at GNC and have received membership rewards and physical results every time. Why did I try Shredz? Because their marketing is genius. I definitely do not recommend Shredz to anyone with a medical condition such as upper respiratory, heart, digestive or sleeping disorders. Your heart will go insane, your stomach will hurt, your breathing won't feel like enough, for as much as your heart is throbbing, and with as much caffeine in your system, you could run across the world in 2 hours without stopping.