L-cysteine and glycine

Glycine and L-cysteine are both amino acids the body uses to build glutathione, a small molecule made of three amino acids working together — which is the actual reason the two show up in the same searches. No published human trial has tested plain L-cysteine plus glycine as a combined supplement, though a related combination (glycine plus a different cysteine-derived compound) has been tested once, with a mostly null result.

Why they're grouped

Glutathione is built from three amino acids: glutamate, cysteine and glycine. A biochemistry review of the sulfur-containing amino acids describes cysteine as the one that gets converted into glutathione (and into taurine) in the body, alongside its separate role forming the disulfide bonds that give structure to proteins like hair keratin. Glycine is the third building block in that same molecule. That shared role in one pathway — not a tested combination — is why the two ingredients appear side by side in supplement marketing and in search.

What's actually been tested together

The closest human data comes from a 2022 randomized trial, but it didn't use plain L-cysteine — it used glycine plus N-acetylcysteine (NAC), a related but chemically distinct, acetylated form of cysteine. Over 2 weeks, 114 healthy older adults took placebo or glycine-plus-NAC at 2.4, 4.8 or 7.2 grams a day combined. The supplement was safe and well tolerated, but its pre-specified main result was null: total glutathione and the ratio of reduced to oxidized glutathione weren't significantly different from placebo at any dose by the end of the study. A post-hoc subgroup — people who started with high oxidative stress and low glutathione — did show more glutathione generation at the medium and high doses, but that's a secondary, after-the-fact finding, not the trial's answer to its own question.

That result is about NAC, not plain L-cysteine, and the two aren't interchangeable — see L-cysteine and NAC: are they the same thing? for why. We found no trial that combined glycine with free L-cysteine specifically, at any dose, for any outcome. If anything, this NAC trial is a reason for caution about assuming a glycine pairing "boosts" glutathione — even the better-studied combination missed its own primary target in healthy adults.

A separate, unrelated search

Some searches that pair "glycine" and "L-cysteine" are actually about a different product entirely: an injectable compound combining glycyrrhizic acid, glycine and cysteine, used as a prescription medical treatment in some countries outside the US. That's an intravenous drug administered under medical supervision, not an oral supplement, and this page doesn't cover it.

Questions people ask

Does taking L-cysteine with glycine boost glutathione more than L-cysteine alone?

No human trial has tested that specific comparison. The nearest data point — glycine combined with NAC, not L-cysteine — missed its own primary glutathione target in healthy older adults, so even the better-studied version doesn't clearly support a "boost."

What's the right ratio of L-cysteine to glycine?

No research has established a ratio for combining the two as supplements. The 2022 glycine-plus-NAC trial used a 1:1 ratio by weight, but that's a different compound and doesn't establish anything for plain L-cysteine.

Related reading

What is L-cysteine?, L-cysteine and NAC: are they the same thing?, and does L-cysteine help with detox and cleanse? cover the glutathione question in more detail.