Neurodegenerative disorders involve a gradual loss of neurons. This includes multiple sclerosis and amyotrophic lateral sclerosis. Yet there is still a shortage of dependable blood-based indicators. These indicators would aid in diagnosis and track therapeutic effectiveness. The neurofilament light chain, which is an essential component of the neuronal cytoskeleton, enters the cerebrospinal fluid and bloodstream in the event of axonal injury. While most previous studies have reported increased neurofilament light chain concentrations in untreated patients, evidence from cohorts that have undergone long-term treatment is scarce. This study measured plasma neurofilament light chain concentrations in 86 participants (33 with multiple sclerosis, 38 with amyotrophic lateral sclerosis and 15 healthy controls) using the Bioassay Technology Laboratory neurofilament light chain ELISA assay. Median plasma neurofilament light chain levels were significantly lower in patients with multiple sclerosis (p=0.0035), whereas the reduction observed in patients with amyotrophic lateral sclerosis did not reach statistical significance (p=0.0728) compared with healthy controls. This suggests a treatment-associated reduction in neuronal damage, despite ongoing neurodegeneration. No significant sex-based differences were detected (p=0.879). The results suggest that plasma neurofilament light chain levels could function not only as an auxiliary tool for diagnosis but also as a marker reflecting therapeutic effects in chronically treated neurodegenerative conditions. A key contribution of this study is the observation that treated patients exhibited lower plasma concentrations than healthy controls, a finding that contrasts with previous reports of elevated levels in untreated groups. This emphasizes the potential clinical utility of plasma neurofilament light chain monitoring and sets the stage for future longitudinal validation efforts.