Some unwanted effects in some people, especially at high doses (~3 mg/day or more) may include: headaches, nausea, next-day grogginessor irritability, hormone fluctuations, vivid dreams or nightmares,[65] reduced blood flow, and hypothermia.[66]
While no large, long-term studies that might reveal side-effects have been conducted, there do exist case reports about patients having taken melatonin for months.[67]
Melatonin can cause somnolence(drowsiness), and, therefore, caution should be shown when driving, operating machinery, etc.
In individuals with auto-immune disorders, there is conflicting evidence that melatonin supplementation may either ameliorate or exacerbate symptoms due to immunomodulation.[68][69]
Individuals experiencing orthostatic intolerance, a cardiovascular condition that results in reduced blood pressure and blood flow to the brain when a person stands, may experience a worsening of symptoms when taking melatonin supplements, a study at Penn State College of Medicine's Milton S. Hershey Medical Center suggests. Melatonin can exacerbate symptoms by reducing nerve activity in those experiencing the condition, the study found.[70]
The use of melatonin derived from animal pineal tissue may carry the risk of contamination or the means of transmitting viral material. The synthetic form of this medication does not carry this risk.[4][71]