In my past pregnancies, phenergan only worked for a short while, and then stopped working. Plus, it makes you really sleepy. Last time I stopped taking Zofran, b/c the constipation (side effect) was REALLY, REALLY bad. This time, I am even doing worse than my other two pregnancies regarding the n/v. So, I asked for a zofran script again (thankfully, it is now offered in generic, the name brand stuff cost an arm and a leg), BUT I am also taking 2 colace tablets, plus miralax once a day to offset the constipation. I've been on the zofran now for almost 2 wks and while I am vomiting less, I still often feel nauseous. I don't think that zofran really completely takes away the icky feeling, BUT I think that if you are throwing up a lot, it will help you throw up less, which will help prevent you from becoming dehydrated. Also, have you tried unisom/b6? I take one unisom tablet (don't get the gel tablet, it is different) before bedtime and that seems to keep my nausea at bay overnight and then I take zofran close to lunch time. I may have to add another zofran though (you can take up to 32 mg a day, right now I am taking one 8mg pill), b/c by the late afternoon and early evening I feel really awful and usually end up throwing up anyway.
I hope you find something that works for you.