Evidence explainerNAC (N-acetylcysteine)

NAC and Glutathione: What GlyNAC Trials Actually Measured

NAC is sold as a glutathione builder, and GlyNAC pairs it with glycine. The largest GlyNAC trial found no overall rise in glutathione; a smaller one reported broad changes.

By Kymata Health editorial teamPublished October 9, 2026Last reviewed October 9, 2026

Medical review pending. Written by our editorial team from the studies cited below; a credentialed clinician has not reviewed it yet. Educational only, not medical advice.

Key takeaways

  • Glycine and cysteine are both required to make glutathione; NAC supplies the cysteine [1][3].
  • In 114 healthy older adults, 2 weeks of GlyNAC at 2.4, 4.8 or 7.2 g/day did not raise glutathione overall [1].
  • A post-hoc subgroup with high oxidative stress and low starting glutathione did show more glutathione on the 4.8 and 7.2 g doses [1].
  • A 16-week trial in 24 older adults reported improvements across many measures with GlyNAC, with 12 people per group [2].
  • Across 20 exercise studies, NAC raised glutathione and lowered soreness and lactate after exertion [3].

Safety at a glance

From our NAC (N-acetylcysteine) monograph. Published interactions and cautions only, each with its source.

Interactions

  • Nitroglycerin (intravenous). In a double-blind trial of 46 patients with severe unstable angina, intravenous NAC 5 g every 6 hours added to intravenous nitroglycerin led to symptomatic hypotension in 7 of 24 patients versus 0 of 22 on nitroglycerin alone. Anyone on nitrate therapy should check with their prescriber before adding NAC.[9]

Who should be careful

  • Regulatory status. NAC was approved as a new drug in 1963, so FDA considers it excluded from the dietary-supplement definition. Its 2022 guidance says FDA intends not to act against NAC products labelled as supplements, but this does not cover products that make disease claims. FDA's initial review found no safety concerns.[1]
  • Psychiatric or lung conditions. Trials studied NAC alongside prescribed care; it is not a replacement for it.[2][4]
  • Before you start. Talk to your pharmacist or clinician before adding this supplement, especially if you take prescription medicines, are pregnant or breastfeeding, or have a chronic health condition.[1]

Full NAC (N-acetylcysteine) safety section

The evidence, graded

Pulled from our monograph evidence tables. Grades follow our evidence-grading policy.

  • Grade CNo effectGlutathione status in healthy older adults (GlyNAC)NAC (N-acetylcysteine) · RCT · n=114 · 2022 · PMID 35821844GlyNAC was safe and well tolerated but did not raise the reduced-to-oxidized glutathione ratio or total glutathione overall. In a subgroup with high oxidative stress and low baseline glutathione, the 4.8 and 7.2 g doses increased glutathione.
  • Grade DImprovedGlutathione, oxidative stress and physical function in older adults (GlyNAC)NAC (N-acetylcysteine) · RCT · n=24 · 2023 · PMID 35975308GlyNAC, but not placebo, improved glutathione deficiency, oxidative stress, mitochondrial function, inflammation, insulin resistance, physical function, waist circumference and systolic blood pressure, and was well tolerated.
  • Grade DNo effectRepeated 4-minute maximal cycling performanceNAC (N-acetylcysteine) · Crossover RCT · n=11 · 2019 · PMID 31030296Mean power did not differ between NAC and placebo in either effort, and plasma antioxidant capacity was unchanged.

How NAC connects to glutathione

Glycine and cysteine are amino acids required to generate glutathione, an intracellular tripeptide that neutralizes reactive oxygen species [1]. NAC's action is closely tied to providing cysteine for glutathione synthesis [3].

GlyNAC combines glycine and NAC, given at a 1:1 ratio in the largest trial [1].

The largest GlyNAC trial: no overall rise

A 2022 randomized trial gave 114 healthy volunteers (mean age 65) GlyNAC at 2.4, 4.8 or 7.2 g/day for 2 weeks [1]. At baseline they had more oxidative-stress markers and a lower reduced-to-oxidized glutathione ratio than a young reference group (11.78 vs 15.26) [1].

GlyNAC was safe and well tolerated, but it did not raise the glutathione ratio (12.49 on placebo vs 12.65 on 7.2 g) or total glutathione, the study's primary endpoint [1]. In a post-hoc subgroup with high oxidative stress and low baseline glutathione, the medium and high doses did increase glutathione [1].

The 16-week trial: broad results, small sample

A 2023 trial randomized 24 older adults to GlyNAC or an alanine placebo for 16 weeks, with 12 young adults taking GlyNAC for 2 weeks as a comparison [2]. GlyNAC, and not placebo, improved glutathione deficiency, oxidative stress, mitochondrial function, inflammation, insulin resistance, physical function, waist circumference and systolic blood pressure, and was well tolerated [2].

The abstract does not state the dose, and many outcomes were measured in 12 people per group [2]. Because the trials test a combination, NAC's own contribution cannot be separated [1][2].

NAC and exercise

A 2024 meta-analysis of 20 studies found NAC lowered muscle soreness and lactate after exercise (lactate by about 0.56 mmol/L), lowered IL-6 and TBARS and raised glutathione, with no effect on creatine kinase, TNF-α or oxidized glutathione [3].

Lower soreness did not translate into speed in the one performance trial in our sources: in 11 well-trained cyclists, NAC at 20 mg/kg did not change mean power in two 4-minute efforts 90 minutes apart [4].

Quick answers

What GlyNAC doses were studied?

The 2-week trial tested 2.4, 4.8 and 7.2 g/day at a 1:1 glycine-to-NAC ratio [1]. The 16-week trial abstract does not state a dose [2].

Is GlyNAC better than NAC alone?

Neither GlyNAC trial included an NAC-only group, so our sources cannot answer that [1][2].

Should NAC and glutathione be taken together?

None of the trials in our sources tested NAC alongside a glutathione supplement; GlyNAC pairs NAC with glycine instead [1].

Can NAC be sold as a supplement?

FDA considers NAC excluded from the supplement definition but said in August 2022 it would exercise enforcement discretion for most NAC supplements [5]. See is NAC safe to take daily.

Keep reading

Full monographs

Related articles

Browse the full Supplement & Nutrient Encyclopedia or all supplement articles.

References

  1. [1]A Randomized Controlled Clinical Trial in Healthy Older Adults to Determine Efficacy of Glycine and N-Acetylcysteine Supplementation on Glutathione Redox Status and Oxidative Damage. Front Aging, 2022. PMID 35821844
  2. [2]Supplementing Glycine and N-Acetylcysteine (GlyNAC) in Older Adults Improves Glutathione Deficiency, Oxidative Stress, Mitochondrial Dysfunction, Inflammation, Physical Function, and Aging Hallmarks: A Randomized Clinical Trial. J Gerontol A Biol Sci Med Sci, 2023. PMID 35975308
  3. [3]The impact of N-acetylcysteine on lactate, biomarkers of oxidative stress, immune response, and muscle damage: A systematic review and meta-analysis. J Cell Mol Med, 2024. PMID 39632267
  4. [4]N-Acetyl cysteine does not improve repeated intense endurance cycling performance of well-trained cyclists. Eur J Appl Physiol, 2019. PMID 31030296
  5. [5]Guidance for Industry: Policy Regarding N-acetyl-L-cysteine. U.S. Food and Drug Administration, 2022.

NAC (N-acetylcysteine) and other dietary supplements are not intended to diagnose, treat, cure or prevent any disease, and these statements have not been evaluated by the FDA. Talk to your clinician or pharmacist before starting a supplement, especially if you take prescription medicines. This page has no product or affiliate links.

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