My dog takes metoclopramide! Help!?

Well my dog is in critical condition at the moment and we have him home and giving him meds but he pukes up white foamy flem a couple hours after taking the metoclopramide and that med is supposed to help him not puke! Why is he puking?? He was fine off of it. But he did go through surgery that hes taking penicillin and that metoclopramide is supposed to help his intestines go back to normal. He takes like 2 pills at different times. And 2 shots at different times. And has to be giving water through an IV in the morning an fed every hour and a half. We have to force feed him! But why is he puking? And yes the vet knows hes puking.. No stupid answers please!