It can really mess up your liver and I'd at least try not to have it within 24 hours of a blood test because they normally check glucose levels and blood sugar spikes can lead to crashes that can may take a while to level out depending on the person.
Also, the irony is that energy drinks, like all caffeinated products, are useless for energy. It becomes maintainance rather than a proper boost. If you have one when you never do then yeah, you get a rush. But overall those who drink caffeinated drinks don't have more energy than those who don't and are actually more likely to have less, especially with how much it affects sleep, or at least the quality of sleep. You build a fast tolerance and so from there you're merely returning your body to a normal range after withdrawl with each caffeinated drink. The more you drink, the worse the withdrawl and the faster your tolerance builds, until it takes even more drinks to get you back to the same levels of that of a someone who doesn't take caffeine products. It's a viscous cycle.
The best way to have energy is exercise. 20 mins of exercise every day will give you more energy than a pack of red bull but without the shakes and crash. It sounds counterproductive but it's true. With exercise you increase your mental focus and can work more efficiently for longer, which more than makes up for the time busy students think they'll lose in bothering to exercise.
