15] Metaphorically, think of your brain as the engine of a car. Certain substances floor the gas pedal and make the car go faster than normal. This is fun. But in driving faster, you use up gas [serotonin, in this specific situation] faster as well.
When these substances wear off, you are obviously left with less gas than you would have if you didn't do the speed-racer thing.
With less gas, you can't drive as fast as you normally do. Therefore you operate less efficiently. Low levels of serotonin can be characterized by changes in eating patterns, sleeping patterns, short term memory and the appearance of depressive symptoms. MDMA produces its loved up, empathic effect by increasing levels of serotonin. If the body's "gas" [serotonin] is used up faster than it is replaced, then one is left with less serotonin when it is all said and done... ...unless you go to a gas station and refill the tank [supplement with 5-HTP, eat foods rich in the amino acid trytophan like bananas or turkey meat]...
...or put the car in economy mode to use the gas more efficiently [use of an SSRI type anti-depressant]...
...or don't drive as fast, as often [use moderation in usage / frequency].
Because this is purely mathematical it applies to everyone regardless of their prior condition.
It should however be noted that the lower levels of serotonin are not permanent [i.e. the size of your gas tank has not been reduced], but rather there is temporarily less gas in the tank than normally.
Damage may even occur to the engine if driven hard with no gas in the tank. Depletion of serotonin stores is one step in a proposed mechanism for the neurotoxicity of MDMA. It can be theoretically counteracted by using engine oil [antioxidants such as Vitamin C]. I must however warn that studies correlating MDMA with neurotoxicity have only been performed in animals and only using ridiculously high dosages / frequency of usage.
There are ways in which one can decrease the negative side effects of MDMA and increase the positive ones. Although no studies have been done on these techniques and MDMA specifically, extrapolating biochemical facts and observing the experiences of those who have done it support their practice. These practices are called preloading and postloading. The entire concept behind preloading and postloading is, extending the above metaphor, adding gas to the tank that MDMA is going to use up.
Serotonin as aforementioned is responsible for the empathic, loved-up feelings from MDMA. Adding more serotonin to your system enhances these effects. More importantly, it hypothetically counteracts the mechanism MDMA is thought to induce neurotoxicity with. 5-hydroxytrytophan is the direct precursor of serotonin in the body. It is sold in health food stores to help people with sleeping disorders [one of serotonin's roles is in the regulation of sleep, as aforementioned]. It is not a drug, it is an amino acid with a hydroxyl molecule appended to it. Supplementation with 5-hydroxytrytophan enhances the loved-up, empathic feelings and cleans up the comedown.
Hard come downs are usually generated by depletion of intraneuronal serotonin stores. The result of lessed serotonin stores, as mentioned above, are characterized by disruptions in normal sleeping and eating patterns, difficulty in verbal processing and committing facts to short term memory, as well as irritatiblity. Most notably the impact of low serotonin levels is characterized by depression. Supplementation with 5-hydroxtrytophan [5-HTP for short] counteracts these negative side effects by putting more gas in the tank and ensuring that you don't run out.
Hope that helps - there's lots of interesting stuff in that FAQ that might assist.
BigTrancer