I was actually prescribed 8mg twice a day. It can def cause headaches, as texasangel said, but you can take Tylenol for those. I am having a boy and for some reason, when I have boys, my headaches are worse. I think the combination of the Zofran and my hormones caused major headaches, but Zofran is def not optional for me!
Either way - watch out for major headaches! At least for me they were...but I'll take them any day over the nausea!
I have been told at different times in my pregnancy that it is 4mg every 4 hours and then 4 mg every 8 hours so not sure why it varies. I would check with your dr and see what they say.
I imagine it would depend on said doctor, and how many times a day. I know for me I was taking 4 mg every 4 hours or so, along with 25 mg of Phenergan. You should ask your doctor though if you're unsure what you're taking, or if you feel it's not enough. We can't really say what's appropriate, because your case might be very different from anothers.