I don't know about the chances, but this is what it says about antiemetics in Stan's Guide:
"The second most important step of the method after making the SN drink and swallowing it, is for the SN to stay in your stomach. You are drinking a toxic liquid and your body defences will recognise this and may try to cleanse your body. One of the defence systems is vomiting.
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Without keeping the SN in your stomach, it will not work. There have been reports from people who followed the correct dosage of an antiemetic and still vomited so there is no guarantee. I would suggest that you do everything possible not to vomit. You do not want any antiemetic though. Buying one for travel sickness for example works on the wrong part of the brain for the SN method. What you need is a dopamine blocker which have been listed above. This is the medication that is most likely to react with people who are on existing prescriptions. Please check the interaction for your own safety.
Some people find it easy to drink something salty with no thought, some may dry retch as part of the natural reaction from the body.
If you have decided to use a particular antiemetic, it is strongly advised to take a test dose before you think of doing the SN method to see if it has a negative reaction with you. This is to avoid any unwanted surprises on the day that you take the SN."