All Perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as that of his family and all his companions.
As for as we know, there is no scientific evidence proving that caffeine, if it is used in a large quantity, becomes intoxicating. If this fact is proven to be true then we would have ruled that caffeine is prohibited, based on the saying of the Prophet : "If a large amount of something causes drunkenness, then any little amount of that thing is forbidden." [Abu Daawood and others]
However, prohibiting something due to it being harmful or intoxicating can not be based on mere assumption or guess or rumors unless it is proven in a way that is accepted in Islam. Moreover, in principle all foods are lawful as it is stated by Al-Hajjaawi, from the Hambali school: "In principle all foods are lawful." The evidence for this is the Saying of Allaah (which means): {It is He who created for you all of that which is on earth.}[Quran 2:29].
Therefore, consuming any drink that contains caffeine is basically lawful. For details see Fatwa 84417.
Allaah Knows best.