The Central Nervous System Modulatory Activities of N-Acetylcysteine: A Synthesis of Two Decades of Evidence.

N-acetylcysteine (NAC) has garnered increasing interest for its neurotherapeutic capabilities beyond its recognized functions as a mucolytic agent and an antidote for acetaminophen toxicity. This review consolidates findings from both preclinical and clinical studies to investigate NAC's diverse modulatory effects on the central nervous system (CNS). NAC primarily functions as an antioxidant by replenishing glutathione and mitigating oxidative stress; however, it produces glutathione-independent effects through the modulation of mitochondrial redox systems, ferroptosis, and the Nrf2-ARE signaling pathway. It plays a significant role in neuroinflammatory processes by inhibiting the production of cytokines, the expression of iNOS, and the activation of microglia. Furthermore, NAC affects various neurotransmitter systems-including glutamatergic, dopaminergic, GABAergic, serotonergic, cholinergic, and adrenergic pathways-by modulating synaptic transmission, receptor activity, and transporter functionality. It promotes neuroprotection through the enhancement of neurotrophic factors, the preservation of mitochondrial integrity, and the upregulation of survival signaling pathways. Recent evidence also emphasizes NAC's role in gene expression and the regulation of cortisol levels. The extensive range of NAC's neurobiological effects highlights its therapeutic potential in treating neurodegenerative and neuropsychiatric disorders. Nevertheless, the variability in clinical outcomes indicates a pressing need for more focused, mechanism-based research.