Why do antibiotics make me constipated?

Altered flora: Diarrhea is much more common with antibiotics than constipation. However, any antibiotic can affect the good bacteria in the large intestine thus affecting gastrointestinal transit.

Flora and fauna: Antibiotics kill good bacteria as well as bad. The gut requires "normal" bacteria to function properly. When they are reduced with antibiotic use, problems can occur. I always suggest the use of probiotics whenever antibiotics are taken to try to prevent this from happening.

Generally no: Most antibiotics, if they cause any GI side effects, it is generally going to cause diarrhea, not constipation.

In general: In general, antibiotics have a common adverse effect of causing loosening of stools or diarrhea...Although anything is possible!