I started to take these pills about two days ago. I\'m a heavy coffee and tea drinker, so I knew I wasn\'t sensitive to caffeine. However, I stopped my caffeine consumption as I started to take Zantrex 3 hoping that it would substitute it. Sure enough, Zantrex 3 gave all the energy that I needed. However, it is a pretty powerful pill, because I only took two pills in the morning and it lasted through the night. Remember I take 4 to 5 cups of coffee everyday to prevent me from falling a sleep, but with two pill of zantrex 3 I NEVER felt tired. Even at night time, I had to force myself to sleep. So if you are not a heavy coffee drinker I am, I would suggest to take VERY VERY small amounts of this pill.