I saw Bush on TV today and his jaw was jerking and jutting as usual. Sometimes I think it's going to come unhinged. It jerks to the left most often and occasionally it juts forward as he pauses between words.
Sometimes he can almost stop the involuntary jaw movements by clamping his lips together during a pause, but even then his jaw sometimes manages to jerk a little bit.
It's quite fascinating so I conducted a Google search and found that this is a side effect of drug use. Drugs prescribed for the treatment of schizophrenia can cause involuntary jaw movements, as can amphetamines and the drug ecstasy.
I found the following in Alcohol Handbook: Chapter 11:
"The effects of ecstasy are quite similar to those found with amphetamines, although it is reported that ecstasy produces a more positive mood and sense of intimacy than amphetamines.
PHYSICAL EFFECTS: Involuntary jaw movements; jaw clenching; teeth grinding "