There seem to be many different causes of Migraine but every doctor I've seen has told me that the biological process that is widely thought to lead to the symptoms is a spasming of a particular blood vessel that runs from the spine/neck to your head (sorry if you already know that!).

Some people can easily highlight the causes of their migraines and do pretty well at managing them, e.g. hormones or foods like nuts/chocolate can be triggers.

Some, including myself, suffer them simply through stress and being both mentally and physically tense (hence the link to the neck area). I only suffer a handful of migraines a year but mine are terrifying - numbness on one side, completely incoherent, often sick, can't see. I usually manage to avoid them through exercise and meditation, as I've yet to try a medication that prevented them.

Some people I guess just get them and finding a root cause or trigger can be hopeless.

You're lucky you've found something that works for you - is there any reason you can't get a repeat prescription to keep stocked up? Would the doctor rearrange your appointment if you tell them you're running too low? There's also a chance that the medication will still have a positive effect after you run out as it'll still be in your system for a short time