メラトニンの一日あたりの推奨摂取量は、5mgでは1錠〜1.5錠となっています。
あくまで参考ではありますが、メラトニンの毒性テストでは、
・ 一日あたり6g（つまり6,000mg）で副作用が見られなかった
・ オランダでは1400人の女性を使い、1日75mgのメラトニンを4年間服用して副作用が見られなかった
という報告があります。
↓メラトニンの毒性テストについて
Is melatonin safe?
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.