Molly is supposed to by pure MDMA, as opposed to pressed ecstasy tablets. Effects include stimulation, mood lift, euphoria, heightened senses especially with sound, loss of appetite, jaw clenching, sweating, difficulty sleeping until several hours after coming down.
If it is pure there isn't much danger, death as a result of direct pharmacological action of MDMA is very rare. Most 'ecstasy' related deaths are from polydrug use, bad pills, water intoxication/hyponatremia or heat stroke.
The solution to these risks; don't do any other drugs besides maybe cannabis with ecstasy your first time. Keep hydrated with sports drinks that contain electrolytes (any will do) but do not drink any more than usual. Do not be overly active and if you feel hot or faint move to a cool place.

MDMA is also contraindicated with some prescription medications such as anti-depressants and MAOI's.