anonandlikeit, re: the headaches, the companies are obliged to list any reported symptoms. This means that even if a headache is incidental, if it has been reported, it has to be listed.
DD hasn't seemed anything other than refreshed when she has awoken, and I think that as it is a naturally occuring substance, it is just giving a 'boost' to the sleepy signal, rather than affecting the brain in any way that overrides natural mechanisms.
I know that for DD, if she is determined in the night to be awake, no amount of Melatonin will convince her otherwise. Where it is useful is in helping her to realise she is tired in the evening at bedtime. I think her brain just gets overwhelmed without the melatonin, and the signal can't get through, so she needs something to shout "YOU'RE TIRED" through the background noise.
With regards to prescriptions, our Paed prescribed 2 weeks' worth of melatonin, and wrote to our GP requesting that he continues the prescription. This covers the GP, who shouldn't prescribe unlicenced drugs if he is not specialist in the area.
Likewise her Epilim, it is prescribed according to the latest Paed letter by the GP, but we adjust the dose in response to our clinic appointments, and ignore what the label says on the bottle.