Osteoarthritis (OA) is a very common and painful condition. Medicines currently available for treating OA pain are not ideal: they are either inadequately effective or cause unpleasant or dangerous side effects. Recent research has shown how the brain processes pain in OA and this has opened up the possibility of using different types of medicines for OA pain. Nortriptyline (an antidepressant) has been used to treat persistent pain in other conditions, and other antidepressants may reduce pain in knee OA. It is not known whether nortriptyline is useful in this condition. We plan to test this effect by randomly allocating participants to treatment with nortriptyline or placebo, and to measure changes in their pain before and after a period on the medication. We hope that this will tell us whether nortriptyline will be helpful. If it is, then we believe that many people may benefit from taking this medicine.