Sonnenblume took metoclopramide. I don't know the exact amount, but probably around 40 mg. At Dignitas, they take 30 mg metoclopramide before drinking N.
Anti-emetics are recommended when CTBing with any ingested substance, whether it's N, SN, opiates, tricyclic antidepresssants, etc. Throwing up is the body's natural defense reaction when taking a lethal substance. Even people who OD from injecting heroin usually throw up.
I have experience with D2-receptor antagonist anti-emetics since I've tried them in the past for a digestive condition. The most commonly-used medications in this class are metoclopramide and domperidone, but theoretically any D2-receptor antagonist should work. (All antipsychotics are D2-antagonists.) Metoclopramide is also a 5HT3-receptor antagonist, so it has 2 modes of function.
The various euthanasia guides have different recommendations when it comes to anti-emetics, so it's not an exact science. Some guides even say that you should start taking them 48 hours before CTBing, but I don't think that's necessary. At Dignitas, they take a single dose an hour before drinking N.
The typical dosages for people who take metoclopramide and domperidone for medical reasons is 10-20 mg 3x per day, so up to 60 mg per day. So you can use that as a general guideline.