Caffeine based drinks such as Relentless/ Redbull/ Monster/ Kick ect are extreemly bad for you. The caffeine in them doesnt provide energy it just effectivly raises your heartrate to "pep you up". Thease drinks are good in terms of stopping tiredness on say long journeys where you need to stay awake to drive ect lol. In terms of sport you require sugary glucose rich drink such as Lucozade. This is proven to actually give you energy due to the high levels in sugars.
There was once a girl that died from drinking Redbull. She was playing basketball, semi-pro i think, and drank a can. When she resumed playing her heart rate was elivated so much she had a heart attack and died. Its not uncommon in young people to have an exercising heart rate of over 140 Bpm. and obviously if this is raised by another 30-40 Bpm from drinking a caffeine drink, well, not good.......