Answer: Yes- for any medication or herb it is very possible to overdose. If you have overdosed, you need to see a doctor right away. Symptoms of a melatonin overdose may include headache, drowsiness, and upset stomach.Melatonin is one of the least toxic substances known. People have taken as much as 6 grams (600 to 3000 times the normal dosage) of the substance in carefully monitored studies with no sign of toxicity. Only four complaints regarding melatonin have been report to the FDA (USA's Food and Drug Administration). The only consistent side effect of high doses has been drowsiness and a slower reaction time. In the most extensive clinical trial to date a high dose of 75 milligrams of melatonin per day was given to 1400 women in the Netherlands for up to four years with no ill effects. The FDA reports that in the more than two years melatonin has been available for sale over-the-counter in the United States, no alarming side effects have been reported