Glutathione is a small molecule the body builds inside its own cells from three amino acids. NAC, short for N-acetylcysteine, is not glutathione at all but one of the raw materials for it.
So the question behind “NAC vs glutathione” is really about delivery. Is it better to hand the cell a finished glutathione molecule, or the part it is missing so it can make its own? A handful of human trials have measured each approach, and one small trial tested them side by side.
How each one is supposed to work
Glutathione is a chain of three amino acids: glutamate, cysteine and glycine. Of the three, cysteine is the one that limits how fast cells can make it, as the authors of the head-to-head trial note in their methods [4]. NAC is cysteine with a small chemical cap; after it is absorbed, the liver strips that cap off and uses the cysteine to build new glutathione.
That route is the basis of NAC’s one approved use. Acetadote, an intravenous form of acetylcysteine, is approved to prevent or lessen liver injury after an acetaminophen overdose [1]. Its label says the drug “probably” protects the liver by maintaining or restoring glutathione levels, or by binding the toxic breakdown product of acetaminophen directly.
Swallowing glutathione itself takes the opposite approach, and its weak point is the gut. Digestion can break a three-amino-acid chain into its parts before it is absorbed, which is why whether oral glutathione raises body levels at all was, for years, an open question.
What happens to oral NAC
A 1986 study gave 10 fasting healthy volunteers a single 600 mg dose of NAC, by vein and in four oral forms, on separate days [2]. The oral availability of NAC itself ranged from 6 to 10%, and its late elimination half-life was 2.27 hours.
A low number there is not the failure it looks like. The study measured NAC surviving intact in the blood, and the whole point of NAC is that the gut and liver break it down to cysteine on the way in. What that low figure does tell you is that blood NAC levels are a poor guide to how much cysteine the liver received.
What oral glutathione trials found
Two placebo-controlled trials of oral glutathione in healthy adults reached opposite answers, and the difference between them is mostly time.
In the first, 40 healthy adults took 500 mg of glutathione twice a day, or placebo, for 4 weeks [3]. Glutathione levels in red blood cells did not change, and neither did two urine markers of oxidative damage.
The second ran for 6 months, in 54 non-smoking adults taking 250 mg or 1,000 mg a day, or placebo [5]. On the higher dose, glutathione rose 30 to 35% in red blood cells, plasma and lymphocytes, and 260% in cells scraped from inside the cheek. Levels fell back to baseline within a month of stopping.
Whether a liposomal capsule changes that picture is covered in liposomal glutathione.
The one head-to-head trial
A 2015 crossover trial gave 20 volunteers with metabolic syndrome three products in turn, for 21 days each, with a two-week break between them [4]. The three were NAC at 200 mg a day, oral glutathione at 450 mg a day, and a sublingual glutathione tablet at 450 mg a day, dissolved under the tongue.
| Product and daily dose | Mean change | What the paper tested statistically |
|---|---|---|
| NAC, 200 mg | +27.0 | Not compared with either glutathione form |
| Oral glutathione, 450 mg | −33.41 | Compared with sublingual only |
| Sublingual glutathione, 450 mg | +27.65 | Higher than oral glutathione |
The formal comparison in the paper was sublingual against swallowed glutathione, and the sublingual form came out ahead [4]. The authors also wrote that the sublingual form was superior to NAC, but in their own table the two average changes in plasma glutathione are close, and no test of that pairing is reported.
No adverse events were reported with any of the three products. Two of the four authors worked for the company that makes the sublingual tablet, and that company sponsored the study [4].
NAC with glycine: the GlyNAC trials
One research group has argued that older adults run short of both cysteine and glycine, so it tested the two together as “GlyNAC.” That is a combination, not NAC alone, so its results do not transfer to a plain NAC capsule.
In a 16-week trial, 24 older adults took GlyNAC or an alanine placebo, and 12 younger adults took GlyNAC for 2 weeks as a comparison [6]. The authors report that GlyNAC, and not placebo, corrected low glutathione and improved oxidative stress, inflammation, gait speed and muscle strength. With 12 people per group, those are early findings.
A larger trial of 114 healthy older adults, run by a company that holds patents on GlyNAC, found no change in its primary endpoint after 2 weeks [7]. Total glutathione at the end was 903.5 mg/L on placebo against 959.6 mg/L on the top dose of 7.2 g a day (p = 0.278). A rise appeared only in a subgroup picked after the fact: people with high oxidative stress and low glutathione at the start.
How each one is regulated
NAC sits in an unusual legal spot. The FDA has determined that NAC is excluded from the legal definition of a dietary supplement, because it was approved as a new drug before it was sold as a supplement [8]. In 2022 the agency said it would use enforcement discretion toward NAC products labeled as supplements, and that it was considering a rule to allow them.
Glutathione is sold as a supplement and is also compounded by pharmacies as an injection. It is on the FDA’s 503A Category 1 list, the bulk substances still under evaluation for compounding [9]. What the injection studies show is covered in glutathione injections, and what the liver research says in glutathione for the liver.
What this means
NAC has the stronger mechanism on paper, because it supplies the part cells run short of, and it is the only one of the two with an approved drug use. Swallowed glutathione has the better direct evidence of raising body stores, but only after months, and only in one 54-person trial.
Neither has a trial showing that raising glutathione this way prevents disease or slows aging. For how glutathione pairs with NAD+ in wellness protocols, see NAD and glutathione; for what the trials recorded as side effects, see glutathione side effects.