Glycerin
Glycerin is rated Clear, and it is one of the few pages on this site where the reassuring number survives a hard look: the same 1989 rabbit-ear assay we distrust when it condemns an oil scored glycerin 0, and a 2025 human comedogenicity study used glycerine itself as the negative control.
Humectant with a strong safety record; not associated with clogging pores.
What it is
Glycerin — glycerol, a small three-carbon triol — is the most common humectant in skincare. It is water-soluble and strongly hygroscopic: it draws and holds water, which is why it turns up in so many moisturisers, cleansers, toners, serums and masks. On a label it is Glycerin, sometimes written Glycerine or Glycerol. The Cosmetic Ingredient Review's safety assessment lists its cosmetic functions as humectant and skin-conditioning agent among others, and reports it in use at concentrations up to about 78.5% in leave-on products — an unusually wide band, though a reported maximum use level is a formulation figure, not a demonstration of tolerance across that range. One distinction is worth drawing before anything else, because it is where the label confusion on this ingredient lives. Glycerin is the free triol. The glyceryl esters — Glyceryl Stearate, Glyceryl Stearate SE, Polyglyceryl-3-Diisostearate — are a different class of ingredient: glycerol with fatty acids attached, used as emulsifiers, and some of them carry positive comedogenic scores in the same table that scores glycerin 0. They share a root word and nothing else that matters here. This page is about glycerin, the humectant, not its esters.
Why we rate it Clear
We rate it Clear at Moderate confidence. What is unusual is that the number circulating for it is correct, and it is correct for the right material — which is not something most pages here get to say. The legacy number, and this time it is the actual ingredient. James Fulton's 1989 rabbit-ear survey in the Journal of the Society of Cosmetic Chemists is the source of nearly every comedogenic score in circulation, and we read its Table I off a render of the journal scan rather than a list quoting it. It prints a row for "Glycerin" at a comedogenicity grade of 0 on his 0-to-5 scale — and his earlier 1984 survey, now read, prints Glycerin 0 as well. That row is glycerin itself, not a derivative: the glyceryl esters sit on their own separate rows in the same table and several of them score higher. On Fulton's own published key, a grade of 0 to 1 "is not considered significant," so a 0 sits at the bottom of the scale he treated as noise rather than finding. But a 0 from this assay deserves exactly as much scepticism as a 4 from it, and the rest of this site spends its time saying so. The rabbit ear is a contested, oversensitive model; Fulton called his own survey "not at all definitive." A single reassuring rabbit score is thin evidence, and if it were all we had, this rating would not be Moderate. It is not all we have, and the second source is both human and independent of Fulton. Aich and colleagues published a prospective, randomised, double-blind comedogenic-potential study in CosmoDerma in 2025. Thirty female volunteers (26 analysed) had test materials and controls applied under occlusion to the upper back for roughly a month, with follicular biopsies taken by the cyanoacrylate method and microcomedones graded under light microscopy on a 0-to-3 scale. Glycerine was the study's negative control. Its mean microcomedone grade was 0.42 (SD 0.76); the positive control, coconut oil, produced a mean grade of 1.92 (SD 0.80). This is real human follicular data putting glycerine near the floor of the scale — and it is worth reading closely for what it is and is not. The study was run and funded by Dr. Reddy's Laboratories, whose own moisturiser was the test product; glycerine was not the article under scrutiny but the reference the method was validated against. That makes the sponsor's commercial interest an unlikely influence on this particular reading — though a negative control is still subject to the study's protocol, analysis and reporting choices — but it also means nobody in that study set out to learn anything about glycerine. It was chosen as the negative control because the field already treats it as the reference for a clean result — which reflects an existing expectation, embodied in a method, rather than a fresh interrogation of the material. So the rating rests on three things, none of them a landmark trial: a legacy rabbit-ear 0 for the correct material, a human negative-control reading of 0.42 on a 0-to-3 scale, and glycerin's long and ubiquitous use without a reputation as a pore-clogger. The last of those is consensus and ubiquity, not a measurement. Moderate, and not High, is the honest ceiling: we found no study whose primary question was glycerin's comedogenicity, with glycerine as the test article and counted comedones as the endpoint.
What the evidence doesn’t tell you
No study we identified was designed to measure glycerin's comedogenicity as its primary aim. The human data point is strong for what it is — occluded application to human skin, real follicular biopsies — but it is a negative control, and a negative control's low reading is chosen and expected more than it is discovered. It validates that the assay separates a known-clean material from a known-dirty one; it does not represent an investigator setting out to find whether glycerine clogs pores and reporting what they found. The legacy 0 is a rabbit-ear reading on the Fulton model — ingredient diluted to roughly 10% in propylene glycol, applied to the inner ear of three New Zealand albino rabbits, follicles scored for keratosis under a micrometer. It is the same assay this site discounts when it condemns an oil, and consistency requires discounting it here too: a 0 from a weak model is weak evidence of a 0. Consensus is not evidence, and ubiquity is not a measurement. That glycerin appears in nearly every moisturiser and has for decades is a strong prior, but it is not a study, and the page should not pretend it is one. Irritation is a separate endpoint from comedogenicity, and glycerin is not free of it: undiluted or at very high concentrations it can be irritating to some skin. That is a different question from follicular plugging, and this rating speaks only to pores. An irritant is not a comedogen and a comedogen is not an irritant. The one thread that gets cited against glycerin is microbiological, and it is worth stating exactly what it says. Gribbon and colleagues, in Microbiology in 1993, grew Cutibacterium acnes (then called Propionibacterium acnes) in continuous culture and tested skin lipids and glycerol as nutrient sources; glycerol at 3 mg/ml was a poor carbon and energy source compared with glucose. That is bacterial nutrition in a chemostat, not comedone formation on a face, and "poor source" is the finding, not "fuel." It is a different endpoint from the one this rating is about, and we are not going to inflate it into a paradox. Finally, a rating is a property of an ingredient, not of a finished product. A formula that pairs glycerin with heavier, occlusive materials can still clog pores; the 0 here belongs to the humectant, and the behaviour of any particular product is a separate question this page does not answer.
Where you’ll see it
On a label it is Glycerin (sometimes Glycerine or Glycerol), and it is nearly everywhere: the CIR safety assessment reports it across the full range of leave-on and rinse-off categories at concentrations up to about 78.5%. Because it is so common and often sits high on an ingredient list, its position tells you very little on its own. The distinction to carry away is the one drawn above. Glycerin, the free triol, is this ingredient and it is rated Clear. The glyceryl esters are not: Glyceryl Stearate SE and Polyglyceryl-3- Diisostearate are separate declarations, and the same 1989 table that scored glycerin 0 scored those two 3 and 4. One shared root word covers materials the legacy literature treats very differently.
Sources
Each source says what it actually is. Several widely-cited “sources” for comedogenic ratings are republishing the same 1989 assay, and counting them as independent agreement is how a thin evidence base gets made to look thick.
- Fulton JE. "Comedogenicity and irritancy of commonly used ingredients in skin care products." J Soc Cosmet Chem 40:321-333 (1989) The primary source of nearly every comedogenic number in circulation, read off a render of the journal scan rather than a list quoting it. Method: ingredient at roughly 10% in propylene glycol, inner ear of three New Zealand albino rabbits, five days a week for two weeks, follicular keratosis scored under a micrometer, on a 0-to-5 scale. Its Table I prints a row for "Glycerin" at a comedogenicity grade of 0 — a row for glycerin itself, distinct from the glyceryl esters, which sit on their own rows and score as high as 4. Fulton's own key: 0 to 1 "not considered significant." Cited as the legacy score, and as the same contested rabbit-ear assay this site discounts elsewhere — a reassuring 0 from it is as weak as a condemning 4.
- Aich B, Dhanak GD, Kotak B, et al. "Evaluation of comedogenic potential of a ceramide-based moisturizer (lotion/cream): A prospective, randomized, double-blind, parallel-group comparative study." CosmoDerma 5:28 (2025) A randomised, double-blind human comedogenic-potential study: 30 female volunteers (26 analysed), test materials and controls applied under occlusion to the upper back for about a month, follicular biopsies taken by the cyanoacrylate method and microcomedones graded under light microscopy on a 0-to-3 scale. Glycerine was the NEGATIVE CONTROL and scored a mean grade of 0.42 (SD 0.76); the positive control, coconut oil, a mean grade of 1.92 (SD 0.80). Controls were applied neat under occlusion (0.025 g or ml on a 3×3 cm zone). Genuinely independent of Fulton — though not of commercial sponsorship — and it is human data, but glycerine was the reference the method was validated against, not the article the study set out to investigate, and the study was run and funded by Dr. Reddy's Laboratories, all of whose employees authored it, with the company's own moisturiser (Venusia CeraPlus) as the test product. Cited as the human evidence that glycerine reads near the floor of a follicular assay, with those limits stated.
- Becker LC, Bergfeld WF, Belsito DV, et al. "Safety Assessment of Glycerin as Used in Cosmetics." Int J Toxicol 38(3 Suppl) (2019); CIR Final Report (2014) A Cosmetic Ingredient Review expert-panel SAFETY assessment, cited for what glycerin is and how it is used — humectant and skin-conditioning functions, reported in use at concentrations up to about 78.5% in leave-on products. A safety review is not a comedogenicity study, and we do not use it as one: safety and comedogenicity are different questions, and this document speaks to the first, not the second.
- Gribbon EM, Cunliffe WJ, Holland KT. "Interaction of Propionibacterium acnes with skin lipids in vitro." Microbiology 139(8):1745-1751 (1993) In-vitro microbiology: Cutibacterium acnes (then Propionibacterium acnes) grown in continuous culture under defined conditions, with skin lipids and glycerol tested as nutrient sources. Cited only to state precisely what the "glycerol feeds acne bacteria" thread actually says — glycerol at 3 mg/ml was a POOR carbon and energy source compared with glucose. It is bacterial nutrition in a chemostat, a different endpoint from comedone formation, and its finding runs toward "poor source," not "fuel."
- CosDNA — glycerin ingredient entry An ingredient-lookup site that lists glycerin at a comedogenic rating of 0. Cited as one of the places a reader will have met the number, and characterised as what it is — a republisher of the legacy scores, not an independent measurement.
Others in the same family
Humectants behave similarly enough that the evidence for one is often wrongly read across to the others. These are its structural relatives, not a guess. What the evidence says about humectants as a family ›
Last reviewed 2026-06-24 · How we decide