Can you take diarrhea medicine like Pepto Bismo when on Keflex?

Yes you can. Just put 2 hours inbetween the Keflex and the Pepto. And I mean 2 hours in both directions. I don't think Pepto can neutralize Keflex, but just in case it can keep them separated in your stomach by 2 hours in either direction. Generally things are out of your stomach in 20 minutes. So after 2 hours they should way past the stomach and the duodenum. Don't worry what this means.

You can have both.

See also: https://www.drugs.com/mtm/pepto-bismol.html