Caffeine can give you an energy boost, and can create a small increase in metabolism. 

Caffeine is found in coffee, tea and energy drinks, while you can also get it from pills.

If you want to lose weight, caffeine is a good way to go. The boost in metabolism means you burn fat and calories more quickly, and the energy boost means you can go harder in the gym.

If you want to GAIN MUSCLE, it is equally helpful in terms of giving you an energy boost. However, too much caffeine can make you feel nauseous and 'hyped-up'.

Like many SUPPLEMENTS, it is up to the individual to choose whether or not to take caffeine. Start off on the low end (100-200mg per day) and see how it effects you.

Related Questions:

1. Do I really need to take supplements?
2. What is the best split for muscle gain? Can I gain muscle doing full body splits?