Levodopa is the closest thing we have to a miracle in modern medicine. When people with Parkinson’s disease start treatment with this molecule and its dose is titrated to an optimal level, many symptoms vanish – at least for the duration of its therapeutic action. The effect on the overall function of a patient affected by Parkinson’s symptoms is so marked that “response to levodopa” was enshrined as part of the clinical diagnosis of Parkinson’s disease. Lack of or poor response to levodopa raises the concern that the patient might not have Parkinson’s disease at all.