dehydration. teeth grinding. lowered inhibitions. paranoia. sleep problems. increased anxiety. muscle tension. some people get nauseous/throw up. chills or sweating. increased heart rate

i think the most dangerous side effect is dehydration because body temperature increases suddenly and drastically. ALSO, a lot of people do X while they go out clubbing and they are dancing/exerting a lot of physical energy which increases dehydration. thats why a lot of clubs have "chill rooms" that are air conditioned and have bottled water and stuff!

It depends on if you are talking about actual MDMA (the pure powder) or Ecstacy, which is the powder pressed into a pill (which often contains other amphetamines).. From my knowledge of pure MDMA, I would say that a risk is just not feeling comfortable before you take it and putting yourself in a bad state of mind.. You should be in a relaxed state before taking it.. Bad effects could include increased heart rate, nausea, anxiety, teeth grinding, and insomnia. Dehydration is a proven risk factor!

To add to pasteele's answer: Because so many people hear that dehydration is the riskiest side effect to using X, there have been reports of people who die of OVER-hydration; that is, they become so obsessed with not being an ecstasy dehydration statistic that they actually end up drowning their system with too much water and dying from the imbalance that too much water in the body creates (depleted sodium, etc).

To give an answer semi-relative to the topic, I've heard that the quality of the Ecstasy determines the negative side effects. Some X is mixed with other things (impure), and can be deadly.

Over a long period of time and with intense, MDMA can also create holes in the brain. I saw it on Oprah, so it must be true.