ds1 has been on melatonin for 4 months, it's changed his life (and ours) by curing his awful insomnia. he's now developed hay fever, and needs loratadine for that.
The combination knocks him out (not necessarily a bad thing!) so I suspect he doesn't need both, but I'm a bit anxious / superstitious / irrationally fearful about messing with his magic melatonin even on a trial basis.