As Parkinson's disease
(PD) progresses, levodopa
appears to become less effective in eliminating motor symptoms. Eventually the duration of “on” states becomes shorter and “wearing off” happens sooner than when the person takes a dose of levodopa. In an "off" state, the person becomes very stiff, slow and may even be unable to move for a few minutes. In some persons with PD, the "on-off" fluctuations are somewhat predictable. They know that the effects of levodopa will wear off after about three hours, so they can plan accordingly. For other people, the "on-off" fluctuations are unpredictable, and this of course is the more dangerous state. No one knows why fluctuations are unpredictable in some cases. Motor fluctuations seem to respond to controlled-release forms of levodopa.