are doing so on the strength of work done by Dr. J.S. Hong in which he demonstrated in animal studies that it is as effective as naltrexone in preventing the inflammation response of microglial cells thought to be the culprit in damaging dopamine-producing brain cells. Although naltrexone is relatively inexpensive, it does require a physicians's prescription, and the cough preparations containing DM do not.
Like Steve said, the claims made for naltrexone, even for MS, are that it appears to stop progression of the disease. I assume that the effectiveness we think we have seen with low-dose DM in PD is slowing of disease progression, not a cure.
My progression has been slow, as indicated by very little loss of normal gait, and fairly well-sustained stamina during the five years I have been using DM. I have required only small increases in meds (sinemet and amantadine, supplemented with CoQ10) over that time, and my neurologist is extremely happy with my history so far.
Bear in mind that I was in the earliest stages of PD when diagnosed in 2001, and may still be in what some have called the "honeymoon" period of responsiveness to sinemet.