A little off the main topic....
In basic terms, a stimulant, funnily enough, stimulates the body. This can include increased heart rate, sense of alertness, etc etc. It's just like an amphetamine, just on a smaller scale and not illegal in itself.
If you mix alcohol with a stimulant, the alcohol can have a more pronounced effect more quickly. It can keep you awake longer so you drink more without feeling it too bad. Small-scale speed users can drink people under the table, because whilst they get drunk quickly, they can keep drinking without the effects (tiredness, lack of co-ordination, etc) getting distincively worse. More alcohol consumed = more brain cells potentially killed. Also, once red bull wears off, the effects of alcohol come back and as you are really tanked...
In any case you shouldn't have too much stimulant because it can kill you - simple.
*wonders how to relate this to F1*
An example of alcohol and stimilant gone wrong would be the Kimi at the bar pictures that went around awhile ago...
