Assalam u Alaikum Wa Rahmatullah
In essence all (non-alcoholic) drinks are Halal unless there is some ingredient that is either explicitly Haram or presumed Haram due to evidence.
Energy drinks are designed to provide strength and keep a person awake by the use of caffeine and other stimulants.
The Islamic answer on the use of energy drinks all comes down to the ingredients present in the drink. If all of the ingredients used in an energy drink are Halal then the energy drink would be considered Halal as well but if there is even one ingredient that is considered Haram then the drink would also be considered Haram.
Here are a few ingredients that will make the drink Haram:
1- Alcohol (Any amount of alcohol in a drink will make it Haram. There are two reasons why alcohol would make a drink Haram: 1) intoxication. 2) Nijasah (impurity). Even if some drink does not intoxicate due to minute presence of alcohol, it would still make it Haram because of the element of Nijasah.
2- Haram protein.
3- Haram enzymes.
4- Haram stimulants.
One should take caution regarding those ingredients that are unverified.
Nonetheless, one should keep in mind that in essence all drinks are considered Halal unless there is evidence to prove otherwise.
And Allah knows best.
Mufti Ikram ul Haq
Fatwa Center of America