This study is a double-blind, multicenter randomized controlled trial of doxycycline on osteoarthritis (OA) progression. Our previous research suggests that doxycycline might help prevent or slow OA development by reducing breakdown of cartilage in joints. The target population is one that is at high risk for the development of bilateral knee OA: overweight middle-aged women with unilateral knee OA at baseline. We hypothesize that doxycycline will decrease the severity or rate of progression of OA. We are recruiting 432 study participants across six clinical centers and randomizing them to treatment or placebo groups (N=216/group). Participants will receive either doxycycline (treatment) or placebo for 30 months.