why draw up vistaril first previously demerol?

my instructor asked me the reasons for drawing up vistaril first in the past demerol. the only explanation i can come up next to is that demerol have vials surrounded by different amount that physican usually prescribed such as 25,50,etc. and vistaril comes in one amount so it requirements to be measure first to be accurate. i'm not sure if thats even a correct common sense. can someone let me know their reason for drawing up vistaril before demerol. gratitude Because vistaril settles the stomach, as it is a tranquilizer. On the other hand, demerol make the stomach roil, and makes one want to puke.