Praise be to Allah.
After studying a number of medical reports and articles, and the results of research, we cannot be certain whether these drinks are harmful or not, or about what quantity is harmful, or for which sector of people. These drinks still need further study in order to determine their real effect on people.
But in the light of what we have read about them, most authors warn against them and doubt whether they are healthy. Many doctors have advised that it is important to do without these drinks and to do other healthy things instead such as exercise, healthy meals, getting enough sleep, and so on.
The warning is more serious for those who have heart problems, high blood pressure or are young (under 16 years).
We should take into account the fact that there are hundreds of type of these drinks, which vary in content, and this makes it more complicated to research them, because what may apply to one drink may not apply to another, and so on.
Until there is more research which shows the real nature of these drinks, we cannot issue a ruling as to whether they are halaal or haraam.
If you want to be on the safe side with regard to your health, and heed the warnings of many doctors, that is better.
You can read some of these warnings via the following links [in Arabic]:
Dr. Fahd al-Khudayri from the specialist King Faisal Hospital – Research Department:
An American medical study:
A Saudi report:
Reports which state that they may be o.k., and lists some of their benefits:
Dr. Hasan Sunduqji – Riyadh:
And Allaah knows best.