People with Parkinson's disease commonly suffer a slowing or freezing of movement caused by the death of neurons that make dopamine, a key chemical that allows brain cells to send and receive messages essential to voluntary movements. Patients regain the ability to move, seemingly miraculously, by taking L-DOPA or related drugs that mimic the missing dopamine. After a few years on L-DOPA, however, most patients again lose motor control — but in an opposite way. Instead of too little, there is too much movement, like involuntary nodding and rocking — side effects known as L-DOPA-induced dyskinesias.