on-off phenomenon

[ŏnôf]
n.
  1. A state in the treatment of Parkinson's disease with dopa in which the individual exhibits a rapid fluctuation of akinetic and choreoathetotic movements.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.