It’s not available over-the-counter in some countries, but that may be because of arbitrary reasons. It’s very cheap and very easy to get in the US so it’s not super dangerous. I knew an older person who was taking 10mg every night, prescribed by his doctor. I take 3mg on an empty stomach whenever I feel like I’ll need a more fulfilling night’s sleep, but I don’t do it regularly. It’s not like our melatonin production is natural as it is, artificial light has been messing that up for generations. I wouldn’t worry about it if it really helps you sleep better. The warnings on the bottle are probably over-cautious, too.