S-adenosylmethionine, or SAMe, is a chemical found naturally in the human organism, made from adenosine and L-methionine, and necessary in a few metabolic pathways: transmethylation, transsulfuration, and aminopropylation (source: Wikipedia; September Nineteenth, 2010 C.E.). However, it can also be taken supplementally, and is thus quite an antidepressant -- two studies (1,2) shew its efficacy: it acts swiftly and deftly, and with few unpleasant effects. Among other uses, it is used to synthesize the monoamine neurotransmitters serotonin and dopamine, which affect mood. I found some at a local supplement shop; it's not cheap -- over $1 (Canadian funds) per 200mg pill -- but it works, and one can't put a price on health.
This is surely something to consider before taking a drug with some nasty side-effects, like many antidepressants. I thought I ought to bring it to popular attention. I AM NOT A MEDICAL PROFESSIONAL; THIS IS NOT A PROFESSIONAL MEDICAL OPINION. As always, be careful when taking any drug, and if in doubt, ask a medical doctor and/or pharmacist; if severely depressed, see a psychiatrist at once.