I've been using 4mgs for while now as insomnia is one of the side effects of a med I take - seems to work fine for me - no ill effects.
I have rest days from the meds (Dr approved) so I take rest days from the melatonin too.
Even tho is over the counter in the US I'd still run it past your Dr.
Worth checking this thread out tho:
http://www.soberrecovery.com/forums/...ecautions.html