Mov Disord. 1998 May;13(3):414-7.
A trial of dextromethorphan in parkinsonian patients with motor response complications.

Verhagen Metman L, Blanchet PJ, van den Munckhof P, Del Dotto P, Natté R, Chase TN.

Experimental Therapeutics Branch, National Institute of Neurological Diseases and Stroke, National Institutes of Health, Bethesda, Maryland 20892-1406, USA.

The effects of the NMDA antagonist dextromethorphan (DM) on levodopa-associated dyskinesias and motor fluctuations were studied in patients with advanced Parkinson's disease. During initial open-label dose escalation, 6 of 18 patients reported a beneficial effect at their individually determined optimal DM dose (range, 60-120 mg/day). The 12 remaining patients either experienced reversible side effects, particularly mild drowsiness, or decreased levodopa efficacy, and were therefore excluded from the study. The six responders entered the double-blind, placebo-controlled, crossover study with two 2-week arms separated by 1 week wash-out. On the last day of each arm, motor ratings were performed every 20 minutes for 8 consecutive hours. In addition, motor complications and Activities of Daily Living (ADL) were assessed using the Unified Parkinson's Disease Rating Scale (UPDRS) and patient diaries. With DM, dyskinesias improved by 25% according to physician's ratings and by 40% according to UPDRS interviews, without compromising the anti-Parkinson effect of levodopa. Motor fluctuations and ADL scores also improved significantly. Although the narrow therapeutic index of DM limits its clinical usefulness, these findings support the view that drugs acting to inhibit glutamatergic transmission at the NMDA receptor can ameliorate levodopa-associated motor complications.

PMID: 9613730 [PubMed - indexed for MEDLINE]

Consider this 1998 study a minute knowing of the variation in metabolisms and the two-step nature of DXM. You begin with 18 PWP and give them a dose that would boost all but the fastest metabolizers out of the beneficial zone. Those "fast" guys would be the group of six that made the first cut. The other 12 experienced "decreased l-dopa efficacy". I bet they did at those dosages.
But the six who were able to metabolize fast enough to stay ahead of the wave did have positive results.