Note - testing head of time for possible side-effects is always advised.
Note - If, after using your chosen anti-emetic, vomiting does occur, even if only a small amount, your immediate plans should be abandoned. This is because it is impossible to know what quantity of drug remains in the stomach and for this reason it would be dangerous to proceed.
If this occurs, move quickly and induce further vomiting. Place your fingers down the throat to physically create the vomiting sensation. It is critical to vomit-up as much of the lethal drug as possible. Follow this procedure by drinking a large amount of water. Try to rest until what is left of the ingested lethal drug ingested has been processed by your body. When things are calm, select an alternative suitable date in the future.
If vomiting, or fear of vomiting persists, it may be necessary to look at other end of life methods. The use of an inert gas like nitrogen - which does not involve swallowing and presents no risk of vomiting - may prove more suitable.