MDMA (3,4-methylenedioxymethamphetamine), most commonly known today by the street name ecstasy , is a synthetic entactogen of the phenethylamine family whose primary effect is to stimulate the secretion of large amounts of serotonin as well as dopamine and noradrenaline in the brain, causing a general sense of openness, empathy, energy, euphoria, and well-being. Tactile sensations are enhanced for some users, making general physical contact with others more pleasurable, but contrary to popular mythology, it generally does not have aphrodisiac effects. Its ability to facilitate self-examination with reduced fear has proven useful in some therapeutic settings, leading to its 2001 approval by the United States FDA for testing in patients with post-traumatic stress disorder.

There have been several fatal overdoses of Ecstasy, resulting in hyperthermia and serotonin syndrome. Acute dehydration is a risk among users who are highly physically active and forget to drink water, as the drug may mask one's normal sense of exhaustion and thirst. Also the opposite, "water intoxication" resulting in acute hyponatremia has been reported. By far the biggest danger comes from the fact that other, more dangerous chemicals (such as PMA, DXM or methamphetamine) are either added to ecstasy tablets, or more often simply sold as ecstasy. Long-term effects in humans are largely unknown and the subject of much controversy —particularly with regard to the risks of severe long-term depression as a result of a reduction in the natural production of serotonin.