What we do is, go ahead and give a Phenergan suppository the first time our child vomits then wait about 20-30 min. and then stress dose. But only wait the 20-30 min.  if your child still has good color, not lethargic, etc. If your child doesn’t keep this dose down then give the injection. This is just how we do it and it has worked out well, but you will find what works well for you! Good Luck!