Until now, the reason why the drug levodopa (L-Dopa), which reduces the motor symptoms of Parkinson’s disease, declines in efficacy after a few years’ use has been unknown. A side effect that then often occur is involuntary movements. A Swedish–French collaboration, led from Uppsala University, has now been able to connect the problems with defective metabolism of L-Dopa in the brain. The study is published in Science Advances.