A disease-modifying therapy is the most important unmet medical need in the treatment of Parkinson disease (PD). Laboratory studies have identified many promising candidate agents, but none has been proven to be neuroprotective in PD. A major limitation has been the development of an endpoint that accurately reflects the underlying disease state. This dramatically limits the potential for a new drug being approved as a disease-modifying agent in PD. For the present, the best opportunity to provide patients with PD with a disease-modifying effect is with agents that have been approved for their symptomatic effects. This article reviews currently available drugs for PD and considers the evidence that they might have neuroprotective effects in PD.