What's in OTC NAC
Over-the-counter N-acetylcysteine (NAC) supplements contain the acetylated form of the amino acid L-cysteine as the active ingredient, typically without the additional pharmaceutical excipients found in prescription formulations.
Active Ingredient
- NAC is the N-acetyl derivative of the naturally-occurring amino acid L-cysteine, with the molecular formula C5H9NO3S and a molecular weight of 163.2 1
- The compound is a white crystalline powder that serves as a precursor to cysteine and ultimately glutathione (GSH) production 2, 3
- NAC is naturally found in onions as a plant antioxidant 4
Formulation Differences: Prescription vs. OTC
Prescription NAC formulations (like acetylcysteine solution USP) contain:
- 20% acetylcysteine (200 mg/mL) 1
- Disodium edetate 1
- Sodium hydroxide and/or hydrochloric acid for pH adjustment (range 6.0-7.5) 1
- Water for injection 1
OTC NAC supplements typically contain:
- NAC as the sole active ingredient in tablet, capsule, or powder form 4
- Variable concentrations of silymarin (70-80% in milk thistle products that may be combined with NAC), though these concentrations are not governmentally regulated and may vary considerably between preparations and manufacturers 5
- Various inactive ingredients depending on the manufacturer, as dietary supplements are not subject to the same regulatory standards as prescription medications 5
Important Clinical Caveat
A critical pitfall with OTC NAC products is the lack of governmental regulation of herbal supplements and dietary formulations 5. Unlike prescription acetylcysteine solution which is standardized, OTC products may have:
- Variable NAC concentrations between different manufacturers 5
- Inconsistent quality control 5
- Potential contamination or adulteration 4
This variability is particularly concerning because NAC has been used as a dietary supplement and in cosmetics, with projections showing continued market growth 4. When precise dosing is critical (such as in acetaminophen overdose management), prescription formulations should always be used rather than OTC supplements.