Melatonin is a pretty safe and effective sleep aid especially for long term use.
And melatonin has been used safely for 2 years or more without problems.
However Melatonin can cause some side effects in some people and the side effects of the Melatonin are irritability, stomach cramps, headache, depression, daytime sleepiness and dizziness.
If you start experiencing any of those symptoms when taking Melatonin then it would be a good idea to stop taking it and seek medical advice from your doctor.