Worse than Nothing

Antibiotic No Better For Coughs, Uncomplicated Chest Infections Than No Medication

Amoxicillin, the antibiotic doctors often prescribe for persistent coughs caused by uncomplicated chest infections such as bronchitis, is no more effective at easing symptoms than no medication at all, even in older patients. This was the finding of the largest randomised placebo controlled trial of antibiotics for lower respiratory tract infections (LRTI) done to date.
...First author Paul Little, Professor of Primary Care Research at Southampton, says in a statement:
"Patients given amoxicillin don't recover much quicker or have significantly fewer symptoms."
In fact, he adds, using amoxicillin to treat patients with respiratory infections who don't have pneumonia could not only be ineffective, but might actually harm them.
"Overuse of antibiotics, which is dominated by primary care prescribing, particularly when they are ineffective, can lead to side effects such as diarrhea, rash, vomiting and the development of resistance," he explains.

Antibiotic use is also associated with alterations in the intestinal microbiota.  They kill off certain strains of bacteria and allow others to proliferate.