I'm strongly anti-prescription and want every medicine to be over-the-counter but it's worth noting there usually are reasons behind the prescription status of a substance you should understand before taking it. Most of the people don't know how to take melatonin the right way. You should[1] take 0.3 mg 7 hours before you go to sleep, NOT 10mg right before that like many people do.

[1] https://www.lesswrong.com/posts/E4cKD9iTWHaE7f3AJ/melatonin-...

Also, unless you are really old or travel to different time zones all the time you shouldn't need long-term melatonin supplementation. It's supposed to adjust your inner rhythm so you will naturally fall asleep at the correct time after some days of taking it.