In
this big list of successful SN suicides, most of them mention taking some sort of drugs alongside the SN, but some of them don't specify any, and a few specifically mention not taking an antiemetic. I didn't read the whole thing, but hopefully it's a good enough source to say that CTBing with just SN is possible. I sure hope it's possible, cause it's what I'm planning to do :P
My understanding is that painkillers wouldn't affect your likelihood of dying, just make death more comfortable, and that antiemetics are useful but not necessary. Without an antiemetic, you'd be more likely to vomit, but if you vomit you can drink another glass of the SN mix afterwards to overwhelm your system, basically ingesting SN more quickly than your body can eject it.