From MedPage Today - those who use chronic antibiotics for acne may be more prone to upper respiratory infections. Note that this is a retrospective study, so the data is by no means definitive:

Patients who use antibiotics for more than six weeks at a stretch to medicate their acne have about twice the risk of developing an upper respiratory tract infection, compared with non-users.

antibiotic use was not, however, associated with increased risk for urinary tract infections, according to David H. Margolis, M.D., and colleagues at the University of Pennsylvania Center for Education and Research in Therapeutics here.

"The true clinical importance of our findings, in which patients and practitioners need to balance the risk of these infections with the benefits that patients with acne receive from this therapy, will require further investigation," the investigators wrote in a report in the September issue of Archives of Dermatology.