Overheating
Serotonin helps to regulate body temperature. If you are in a hot place, dance a lot and get dehydrated, ecstasy can cause your body to overheat. To prevent this it is important to cool down once in while. The best way to do this is by sitting down and relax in a well ventilated and cool area.
Water intoxication
Drinking water also helps a bit against overheating. But drinking too much is also risky. When using XTC your body retains more water than usual. In combination with drinking too much water this can lead to water intoxication, which can be lethal.