Here is a link to the University of MD medical center...should be legit with the rules of this site...
http://www.umm.edu/altmed/articles/melaton
in-000315.htm
Precautions:
Because of the potential for side effects and interactions with medications, people should take dietary supplements only under the supervision of a knowledgeable health care provider.
Some people may experience vivid dreams or nightmares when they take melatonin. Overuse or incorrect use of melatonin may disrupt circadian rhythms. Melatonin can cause drowsiness if taken during the day. If you experience morning drowsiness after taking melatonin at night take a lower dose. Additional side effects include stomach cramps, dizziness, headache, irritability, decreased libido, breast enlargement in men (called gynecomastia), and decreased sperm count.
Pregnant or nursing women should not take melatonin as it could interfere with fertility.
Here is another site:
http://www.sltbr.org/melfaq.htm
How safe is melatonin?
Current research suggests it probably is safe to use melatonin if you are:
* over 18 years old, and healthy
* not pregnant
* not breast-feeding
* not taking other medications except for minor analgesics and oral contraceptives
And there are many sites more that you can find on your own. Please do not assume that other posters don't know what they are talking about. If you have reasonable information for this site, do give it without targeting other posters' knowledge and/or expertise.