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.
Molly and Dougall have been taking Cimetidine for 4 years without any problems. I believe this drug has slowed down the progression of Molly's sm. Molly was diagnosed at 18 months, it was predicted she would not be with us by the age of 2 years, Molly will be 6yrs in May.
My cavalier Maisy who has CM takes Cimetidine and Pregabalin, which seem to work well for her. However I am a little concerned now that she may be taking too much antihistimine as she is also an Atarax, an antihistimine prescribed for her pollen and dust allergies. I hadnt realised that cimetidine acts as an antihistimine. I hope she isnt overdosing on them :-(