Background: Multiple sclerosis (MS) is a neurodegenerative disease characterized by neuronal and synaptic loss, resulting in an imbalance of excitatory and inhibitory synaptic transmission and potentially cognitive impairment. Current methods for measuring the excitation/inhibition (E/I) ratio are mostly invasive, but recent research combining neurocomputational modeling with measurements of local field potentials has indicated that the slope with which the power spectrum of neuronal activity captured by electro- and/or magnetoencephalography rolls off, is a non-invasive biomarker of the excitation/inhibition (E/I) ratio. A steeper roll-off is associated with a stronger inhibition. This novel method can be applied to assess the E/I ratio in people with multiple sclerosis (pwMS), detect the effect of medication such as benzodiazepines, and explore its utility as a biomarker. Methods: We recruited 44 healthy control subjects and 95 people with multiple sclerosis (pwMS) who underwent resting-state magnetoencephalographic recordings. The 1/f spectral slope of the neural power spectra was calculated for each subject and for each brain region. Results: As expected, the spectral slope was significantly steeper in pwMS treated with benzodiazepines compared to pwMS not receiving this medication (p = 0.01). In the sub-cohort of pwMS not treated with benzodiazepines, we observed a steeper slope in cognitively impaired pwMS compared to cognitively preserved pwMS (p = 0.01) and healthy subjects (p = 0.02). Furthermore, we observed a significant correlation between 1/f slope and verbal and spatial working memory functioning in the brain regions located in the prefrontal and parietal cortex. Conclusions: In this study, we highlighted the value of the spectral slope, a novel non-invasive biomarker of the E/I ratio, in MS by quantifying the inhibitory effect of benzodiazepines and by putting it forward as a potential biomarker of cognitive deficits in pwMS.