Melatonin Side Effects, Uses, Dosage (Kids/Adults) - Drugs.com In general this medication appears to be relatively nontoxic, even at higher doses such as 3 to 5 mg once a day. Some people can have side effects from melatonin that may include: daytime drowsiness, dizziness, weakness, or confusion. vivid dreams, nightmares. feeling depressed, anxious, irritable. headache.