Synopsis

Source of validity: Scientific
Rating (out of 5): 2

L-cysteine, particularly in its N-acetylcysteine (NAC) form, has some scientific support for use as an adjunct in the management of influenza. NAC is a precursor to the antioxidant glutathione and possesses mucolytic properties, which can help reduce the viscosity of mucus in the respiratory tract. Several studies have examined its potential benefits in viral respiratory infections. A notable double-blind, placebo-controlled clinical trial published in the "European Respiratory Journal" (1997) demonstrated that oral NAC supplementation (600 mg twice daily) significantly reduced the incidence and severity of influenza-like episodes in elderly subjects. Participants experienced fewer symptomatic episodes and less severe symptoms compared to placebo, although the rate of viral infection was similar, suggesting an effect on symptom modulation rather than prevention of infection itself. The proposed mechanisms include reduction of oxidative stress, modulation of inflammatory responses, and improvement of mucociliary clearance in the airways. However, while these findings are promising, NAC/L-cysteine is not a first-line treatment for influenza and should not replace antiviral agents or vaccination. The overall quality and quantity of evidence are limited, and most guidelines do not specifically recommend L-cysteine or its derivatives for influenza. Further large-scale, high-quality trials are needed to better define its role. Thus, while there is modest scientific evidence supporting its adjunctive use, it is not considered a primary or standalone therapy for influenza.

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