I was prescribed this by the hospital for hyperemesis from 6 weeks until 17 weeks after being admitted due to dehydration and starvation. I still felt ill after stopping but I was able to cope with it better. Ondansetron is the generic version of Zofran and I asked my consultant the same question and she said no medication can be described as safe in pregnancy as no pregnant women will submit their pregnant bodies to a medical study (understandably!). I am now 33+4 and my little girl is very healthy and while I've lost a lot of weight in this prehnancy she is measuring large for GA, so very like my first.
I would try not to worry, because your options are to take the medication and be able to eat enough for your safety and your baby's or to suffer from dehydration and starvation which would severely affect you both.
I took the same meds/zofran until about a week ago (17weeks) and it helped a bunch. I found taking it just once a day was plenty to get me thru most days at work and after that I just dealt with it on my own at home. Its ok to take, just dont use more then you need or more then every 6 to 8 hrs
It should be okay if your dr prescribed it.. i take a med for nausea too... dobt know if its working quite yet...