I have tried many of them, since the late 90s, I can't say they worked wonders in terms of keeping me awake when I was tired/sleepy. In fact most of the time I would have one or two and soon after I would go to bed and have a solid sleep. I might, I need to test it further, have found one that actually works with me. It's Monster (the) Red Ultra 500ml can. I have tried it 4-5 times and it just works. I have tried other Monster drinks and they weren't any more effective than other brands, but this version works. I don't know why, I assumed they all are the same...I need to do some further testing.