Does it help to take a probiotic when also taking keflex?

Yes: You can take probiotics, but the Keflex will be counter-balancing them so it will be important to take them after you are done with your course of antibiotics to re-establish your beneficial bacteria in your gut.

Probiotics: Keflex is an antibiotic that kills pathogenic bacteria as well as good bacteria in intestine which will allow growth of bad bacteria in intestine. Probiotics or yogurt when taken with antibiotics will replace these lost gut flora or bacteria and prevent overgrowth of bad bacteria like c.Difficile.