What are the long-term side effects of melatonin?

Question

What are the long-term side effects of taking melatonin as an oral supplement?

The couple of articles I have read are inconclusive:

http://www.huffingtonpost.com/2015/03/11/what-is-melatonin-sleep_n_6795220.html

Although you can't overdose on melatonin, doctors aren't sure whether relying on it can affect you negatively.

http://www.webmd.com/a-to-z-guides/jet-lag-and-melatonin-topic-overview

The long-term side effects of melatonin have not been well studied. If you have epilepsy or are taking blood thinners such as coumadin (Warfarin), talk to your doctor before using melatonin.

Answer 1

In long term studies, the effects have been mixed but mild.

In hemodialysis patients, after about six months - melatonin no longer seemed to have physiologic effects but could still be detected at elevated levels in salivary samples suggesting the body adapted to it.

Long term (six months) melatonin in mice improved spatial learning and memory and also protected against aluminum toxicity suggesting that neuronal or axonal growth is improved with supplementation.

Swedish teenagers and children with ADHD have been placed on melatonin for several years without known adverse effects.

Only mild averse effects have been found with long term use of melatonin in humans (dizziness, headache, nausea, sleepiness.)