I don't see anything at all about concerns with taking the two drugs at the same time. There is no evidence I can find of a biochemical interaction (strictly speaking, based on chemistry alone), the classic sort that raises medical concerns.

This drug has such specificity for a serotonin receptor found only in the gut (5-HT4), which this drug stimulates....I don't see a problem. There is a little bit of cross-binding, which is to say, binding to other serotonin receptor classes. So, there could be mild side effects. Headache, for example. Nothing that stands out in any way.

I think this is a safe combination. Have you read the monograph? http://www.rxlist.com/cgi/generic3/zelnorm.htm

The big side effect to watch for is abdominal pain. That and bloody diarrhea. If you don't have those, I just hope it works as expected.

Take care,
Lar