C4A and Parkinson disease: Results of this study showed that serum complement C4 levels in amphetamine addicts (21.2 ± 0.75 ng/ml) and PD patients (20.62 ± 1.01 ng/ml) were comparable to that in controls (22.73 ± 0.46 ng/ml) (p = 0.191 and p = 0.065, respectively).