The danger in using an expired antibiotic would probably not come from the product, but by the ineffectiveness it would have in fighting off the infection it was prescribed for treatment. This sounds as if you or a friend has some "old" Doxy lying around and would just assume use it rather than getting a new prescription filled. The problem with an expired Rx is that it will have lost some of its potency over time. Using the expired Rx could cause issues with microbial resistance amongst the bacteria inside your body, and eventually leading to having to take an even stronger (and more expensive) antibiotic to get a hold on the infection. It's just safer to use a new, or very recently filled Rx instead.
Source(s):
My source would be many semesters of courses on Microbiology, Immunology, Biochemistry, and Pharmacology in medical school.