Haley plans to take a couple tablets of ecstasy during a dance party that may last the entire night. one of the immediate dangers of using the drug at this event is

The answer to this question is Dehydration

When you take a couple tablets of ecstasy, it will cause a noticable increase in your body temperature.

This will make the liquid substance inside your body to be evaporated from Haley's body and will most likely lead to dehydration.