Do antibiotics cause diarrhea?

Yes. Most antibiotics can cause diarrhea while they are being taken. Some antibiotics can cause an overgrowth of clostridium difficile (also known as "c-diff"), and this causes serious diarrhea, especially in the elderly. This diarrhea can occur as much as 6 weeks after stopping the antibiotic. Patients should notify their physician is they have diarrhea while taking or after taking antibiotics.
Yes. Antibiotics kill the good gut flora and can cause diarrhea. It is a good idea when on antibiotics to take probiotics to replace the gut flora that is killed. One containing a combination of lactobacilli and bifidobacter would be a good start. Also eat foods that help your body make their own gut flora, foods such as bananas which are high in fructooligosaccharides.