Serotonin is a hormone produced by the brain to induce feelings of wellness and happiness. If the brain produces low amounts of these, then you can get depression, mood swings as a result.
Natural serotonin is present in some foods (the most famous one being chocolate) so it is possible to get a natural 'pick me up' from eating these.
In cases of clinical depression/severe hormone imbalance, food does not always do much good and so dr's use artificial serotonin (commonly known as anti-depressants) to replace low levels of this hormone in the brain.
However, although low doses of anti-depressants are usually extremely helpful, high dosage amounts of articificial serotonin can produce anxiety/ panic attacks, hallucinations etc (similar to overdosing on other types of drugs - i.e, you can basically have too much of a good thing).
In extreme cases they can make you violent and agressive and there are mental health groups in the UK who are campaigning to stop dr's giving out high doses of anti-depressents to mildly or moderately depressed people because of the risk of these effects.
They are saying that most people, apart from those with extreme mental health problems, should be given lower dosages and that more attention should be paid to diet and counselling therapies.
That's all I know on serotonin - you can get more info on it from the packet insert contained in drugs which have artificial serotonin in it.
I am pretty sure that sex causes the brain to produce more serotonin also (read it in a women's magazine) but don't quote me on that as I can't remember the name of the mag.
