Just water, nothing else at all. The place where I had mine doesn’t let you leave either so bring some entertainment with you in case. Depending on how your body reacts you might also feel a bit nauseous and tired too. Have some snacks and drink ready in your car or whatever for straight afterwards because you’ll be really thirsty (because you can’t drink at all during the 2hrs either)
Just water
Strictly just water, not even gum