Expired Melatonin is very safe to take and it won't harm you if you do take the expired melatonin.
The only problem with expired melatonin is that it may not work as good as before at helping you sleep but it won't harm your health to take the expired melatonin.
I've taken expired melatonin before and it didn't help me sleep as good but It also didn't hurt me.
If you want to get a good nights sleep then it would be best to get some fresh melatonin to take instead of the expired melatonin.
The same is true for other expired medications.
The expired medications are safe to take but they lose their effectiveness once they get past their expiration date and the older they get.