Beta blockers as preventative treatment?

My GP has again bought up the suggestion that I go on preventative medication for my migraines. The last Time out was discussed I refused because the only option I was given was to take amitriptyline and that would potentially mean decreasing or changing my current antidepressants which I won't do unless it's absolutely necessary because I have severe depression and anxiety that ruled my life for a decade and the current medication is the only one which has made things manageable. I told this to the Dr and she informed me that there were other options and suggested I try beta blockers. Is anyone else taking these for migraine prevention? What kind of side effects do they cause?

Yeah thats what I'm concerned about, my blood pressure has always been on the lower side of normal, but I've got 2 other conditions (Raynauds and Graves) which could also benefit from beta blockers.

How long do they take to kick in? I've got a study break coming up soon and I figure if I'm going to give them a shot, that would be the best time to do it so its not as much as an impact if I do get any side effects

I'm on venlafaxine for depression which is why I can't take amitryptaline for migraines. I might bring it up again with my GP, the migraines are getting a bit nuts again.