None of the ingredients contained in energy drinks are dehydrating. This is also true for caffeine, which is contained in regular energy drinks in the same amount as in one normal cup of coffee. This was confirmed by the European Food Safety Authority (EFSA), the risk assessment body for food safety in the European Union (2015).
However, adequate fluid intake is crucial for the human body, especially during sports and intense exercises. Without adequate fluid intake, intense physical activities may lead to dehydration.
Energy drinks are functional beverages and not sports drinks designed for rehydration. Although they are water-based beverages which provide water to the body, they have not been formulated to deliver rehydration.
In general, it is recommended to drink lots of water during physical exertion, especially during intense exercise.