From Wikipedia ( so it MUST be true Wink )

A version of the drink was marketed for three years in the UK during the late 1990s. Many believe the urban legend that Mountain Dew is illegal (in its American formulation) to sell in the UK due to its high caffeine content and excessive artificial ingredients. However, the version sold in the UK was a lurid green color and contained far more caffeine, synthetic sugars, and artificial colorings than the American version. It even discolored the teeth and tongue of its drinkers for several hours after consumption.

http://en.wikipedia.org/wiki/Mountain_dew

read the rest of the entry! - I'm not going to try drinking it again!