This is very interesting, I have had dogs on both drugs at different times.

The omeprazole has made several of them sick - this happens with people too! As an aside, people are now being taken off Omeprazole as it is now believed to make you more susceptible to Clostridium difficile - diarrhoea, fever, stomach cramps, particularly a problem in hospitals [highly infectious] and can be fatal.

C Diff can affect dogs but I have not heard any advice suggesting they stop taking it for this reason.