A class of antibiotics known as Flourinated Quinolones or simply Flouroquinolones (which included Cipro and Levaquin) appear to cause extremely adverse reactions in some people affecting connective tissue (tendons and ligaments) by interfering with regrowth, regeneration and the natural repair of such tissue. Spontaneous tendon ruptures have been directly linked to the drug, usually occurring within a few days to weeks of usage. While the drug does not affect most people in this manner at normal doses, it does seem to have a devastating effect on individuals who are particularly susceptible. Because of the long time-course of action, ranging from days up to 18 months, it is sometimes difficult to trace connective tissue problems back to use of the drug. Only after much anecdotal evidence was gathered from otherwise normal athletes and individuals who suddenly presented with unexplainable connective tissue problems and who had used the drug in the previous year did doctors see the longer time scale of potential harm that this drug can cause. Also, over protracted periods or many cycles of use, people who were not previously affected start to report symptoms as well. While this class of drugs constitutes extremely strong antibiotics (often used after surgery to prevent infection) in most cases, a different class of antibiotics is equally effective. Some physicians are now advising patients to avoid this class of drugs accept in life or death situations. Effects may be cumulative so even if you have taken one of these drugs previously (even if you did not experience any problems) you probably should avoid future use.
As this is particularly pertinent to athletes, I thought I'd pass it along.