To prevent nausea and vomiting, you can take the non-prescription anti-nausea medicine an hour before taking the first dose of EC. Note that this might make you drowsy.
If you end up vomiting within an hour of taking a dose of EC, you may want to take that dose again in case your body did not absorb the hormones yet.
References:
- FSRH Faculty of Sexual & Reproductive Healthcare. (Amended 2017). FSRH Guideline Emergency Contraception. London. Retrieved from https://www.fsrh.org/standards-and-guidance/documents/ceu-clinical-guidance-emergency-contraception-march-2017/