Doing a literature search, there is a massive amount of research on this booster effect. See Rick's list!!! I could not find any adverse effect on PD patients, but that does not mean there isn't any. I just may not have found it. The mechanism, at least with Sinemet is that grapefruit attacks an enzyme that breaks up levodopa in the bloodstream. The overall effect is as if you had takern a larger dose of Sinemet.
Coincidentally, a full page article was published yesterday on the effect, in the UK Daily Mail. It works with other fruit such as cranberries, some citrus fruits.
With an agonist like cabergoline, see the abstract below.

Effect of grapefruit juice on cabergoline pharmacokinetics in patients with Parkinson's disease
M. Nagai MD, PhD1, A. Nakatsuka1, H. Yabe MD1, T. Moritoyo MD, PhD1 and M. Nomoto MD, PhD1
1Clinical Pharmacology and Therapeutics, Ehime University School of Medicine, Touon, Ehime, Japan
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Abstract
Background: Cabergoline is one of the synthetic ergoline dopamine agonists, which is widely used for the treatment of Parkinson's disease (PD). Cytochrome P-450 (CYP) 3A4 contributes to metabolize Cabergoline. It has been well known that grapefruit juice inhibits CYP3A4 enzyme located in the gut wall. To investigate whether grapefruit juice influences the pharmacokinetics of cabergoline, plasma level of cabergoline in patients of PD was evaluated.
Methods: Five patients with PD treated with cabergoline were enrolled. Plasma concentrations of cabergoline before and after coadministration of grapefruit juice were evaluated. The plasma concentration of cabergoline was determined using a LC/MS/MS.
Results: The plasma concentration of cabergoline increased approximately 1.7 times, when grapefruit juice was taken together with cabergoline. Adverse events were not observed during this trial.
Conclusions: Coadministration of grapefruit juice with cabergoline increases bioavailability of cabergoline. A relatively large therapeutic window of cabergoline may allow the concomitant treatment with grapefruit juice, and this combination treatment may augment the antiparkisonian effect of cabergoline.

I have not tried any more tests at present until I do more searches, but it appears to have real promise of giving a huge boost to our normal drugs. It could enable you to half your regime .
We need to find out what is best for us, drinking a fruit juice with half our normal drugs intake, or taking double that level of drugs.
Ron