Just tell your doc you want to try something else...
I tried prochlorperazine instead because I didn't want to end up like Lil Wayne and my neurologist said they did basically the same. Didn't really like it either tho, but lots of meds if one isn't working they just try something else. Everyone responds differently to medication.
But if it's primarily nausea, it doesn't make sense, when I got prochloro they gave me Benadryl as well for the nausea it caused as a side effect. But even just Dramamine would make more sense then prescribing lean just for nausea.
What ended up helping me was neural stretching and some occasional gabapentin, my nausea was just one of a laundry list of symptoms. After doing the neural tension stretches nausea is mostly gone, and overtime I'm optimistic it will clear everything up. But the gabapentin is a good bandaid for bad days.
Promethazine for nausea just sounds wild, but obviously your doctor knows more about your health than I do, there must be some reason for it.