Butyl Stearate

Butyl stearate is rated Clogging, and the single number the charts carry is the lower of Fulton's two printed readings — 3 on his 0-to-5 scale in 1989 and 4 in 1984, under two different verdict keys — while Nguyen, Dang and Maibach's independent 2007 assay lists it among the materials it deemed positive; no direct result puts it in the insignificant band, and none of them is a human face.

What it is

Butyl stearate is the n-butyl ester of stearic acid, the eighteen-carbon saturated fatty acid, joined to n-butanol. On a label it is Butyl Stearate; its CAS number is 123-95-5 and its formula is C22H44O2. A formulator reaches for it as an emollient: the Cosmetic Ingredient Review describes the seven stearate esters it assessed, this one included, as "either oily liquids or waxy solids that are primarily used in cosmetics as skin emollients at concentrations up to 25 percent." That is a statement about texture and dose; it says nothing about pores either way. Two neighbours are worth keeping separate from it. Isobutyl stearate is a different ester built on a branched butanol rather than the straight chain, with its own identity and its own CIR record; it is not this material. And the stearate soaps — magnesium, zinc, calcium stearate — are metal salts of stearic acid, not butyl esters, and belong to a different part of the label. Fulton 1989 scores several of these separately and they do not agree with each other: zinc stearate 0, magnesium stearate 1, stearyl alcohol 2, stearic acid 2-3*, isopropyl isostearate 5, all on the same 0-to-5 scale. Butyl stearate is the butyl ester specifically, and that is the row to look for in the assays below.

Why we rate it Clogging

We rate it Clogging at Low confidence — Clogging because of what the three readings say, Low because every one of them comes from a rabbit ear. What the page exists to add is that there is not one number. There are three readings of this exact material, two of them Fulton's own, and the one that circulates is the lowest of them. Fulton 1989. His 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 butyl stearate at a comedogenicity grade of 3 on a 0-to-5 scale, with no asterisk, and outside Section VI, Oils — the section whose heading carries Fulton's "results depend on source of raw material" caveat. On his own published key, "a minimal grade of 0 to 1 is not considered significant," 2 to 3 is "borderline," and only "a grade of 4 to 5 is uniformally reproduceable and considered positive." So the 3 sits in his borderline band. Fulton 1984. He ran an earlier survey too — Fulton, Pay and Fulton, in the Journal of the American Academy of Dermatology — and we have read its Table I in full off the journal scan. It prints butyl stearate at 4, unstarred, which by that paper's own preparation footnote means it was applied as is rather than diluted. The 1984 verdict key is not the 1989 one: it says "a minimal grade of 1 to 2 is not considered significant," while "a grading score of 3 or above is uniformly reproducible and considered positive." By that key the 4 is positive, and so is the abstract, which names butyl stearate among the offenders "such as isopropyl myristate and its analogs." The two readings differ by one grade, and it matters which key you read them under. Both numbers sit on the same 0-to-5 scale, one grade apart, and both papers are the same lead author's. Apply the 1984 key to both and both are positive; apply the 1989 key to both and the 4 is positive while the 3 is borderline. There is no key printed in either paper under which either reading is "not considered significant." So this is a disagreement about severity, not about direction, and a bare "butyl stearate is a 3" hides that its author read it a grade higher five years earlier, on a threshold that would have called both readings positive. A third assay, and it is not Fulton's. Nguyen, Dang and Maibach tested cosmetic ingredients in the rabbit external ear canal and published in Cutaneous and Ocular Toxicology in 2007. Butyl stearate is the third of the fourteen materials their abstract lists, and the abstract states, "The first nine were deemed positive." That is a later, independent laboratory reaching a positive classification on the exact named material. We have read the abstract, not the paper: the numerical score, the concentration, the vehicle and the animal count are not in it, its scale is 0 to 4 rather than Fulton's 0 to 5, and we carry no grade from it. A number we have not read is a number we do not print. The one independent laboratory that ran Fulton's own scale never tested it. Morris and Kwan's 1983 survey, in the same journal, is the legacy assay that read materials on 0 to 5 in a different lab and said in print that its results "conflict with the high scores published by Fulton et al." Its thirty-two rows include eleven esters — decyl oleate, isopropyl palmitate, isocetyl stearate, isopropyl isostearate and the rest — and butyl stearate is not one of them. So there is no second 0-to-5 reading to check either of Fulton's against. That is a hole in the record rather than a low score. So the record, stated plainly. Three direct rabbit-ear readings of butyl stearate exist: 4 in 1984 (positive by that paper's key), 3 in 1989 (borderline by that paper's key), and an independent 2007 positive classification whose number we have not seen. None of the three places it in an insignificant band, and we identified no assay, rabbit or human, that reads it as clear. That is why the label is Clogging rather than Caution. It is not the 1989 three on its own, which is borderline by its own key and would sit with the site's other borderline threes; it is that the same author read it a grade higher in 1984, above his own positive line there, and a second laboratory twenty-three years later classed it positive without a Fulton number in front of it. Nothing in the record pulls the other way. The confidence stays Low, and that is the honest half of the sentence — three rabbit ears and no human comedone count is a thin basis for any label, including this one.

What the evidence doesn’t tell you

Every number on this page came off a rabbit's ear, and the three assays that produced them are not replicates of each other. Fulton's 1989 method: the ingredient diluted to roughly 10% in propylene glycol, about 1 mL applied to the inner ear of three New Zealand albino rabbits, five days a week for two weeks, follicles then scored for keratosis under a micrometer. His 1984 method used two rabbits per test, one ear dosed daily for two weeks and the alternate ear as the control, borderline cases confirmed by microscopic biopsy — and, because the butyl stearate row is unstarred, the paper's footnote puts it in the "used as is" group rather than the diluted one. Nguyen, Dang and Maibach used the rabbit external ear canal; beyond that we cannot describe their conditions, because we have only their abstract. Different dilutions, different animal counts, different ear methods, three decades apart. That is not a series of confirmations, and we should not present it as one. Concentration is doing work no single number carries. The 1989 reading is a roughly 10% exposure; the 1984 reading is, by the paper's own rule, an undiluted one; the 2007 exposure is unknown to us. Meanwhile the CIR reports the stearate esters in use at up to 25%. A comedogenic grade is a property of a material at a dose in a vehicle, not a fixed trait it carries into a finished formula — and the author who produced two of these numbers distrusted the reassurance that a low percentage settles it. His 1989 conclusion reads, "We are not convinced of the statement that lower concentrations of these compounds can be safely used with no comedogenic consequences." That cuts both ways at once: against assuming a few percent is harmless, while conceding the safe threshold was never established. Material grade is unrecorded. Butyl stearate's 1989 row carries no asterisk, so Fulton did not flag it as one of the materials whose result depends on the source of the raw material. But none of the three records we hold names a supplier, a purity, a free-stearic-acid content or a lot. Three papers tested something called butyl stearate; whether they tested the same commercial grade is not something any of them lets us check. We are not going to tell you why it clogs. The obvious temptation is a mechanism — that a light ester spreads into the follicle and holds sebum against the wall — and the content mills state exactly that as fact. None of these assays demonstrates a mechanism; they counted follicular keratosis, they did not explain it. A mechanism asserted to dress up a rating is not evidence. Human evidence, stated precisely: we identified no comedogenicity study that applied isolated butyl stearate to human skin and counted comedones. The Cosmetic Ingredient Review's 1985 assessment of the stearate esters concluded they are "safe as cosmetic ingredients in the present practices of use" — a judgement about toxicity, irritation, sensitisation and phototoxicity, not about follicular plugging. It is worth noting that the panel did not treat the two as the same question either: the same abstract states that "comedogenicity is a potential health effect that should be considered when the Stearate ingredients are used in cosmetic formulations." A safety clearance that expressly leaves the pore question open is not a pore result in either direction. The assay tradition has been contested for decades, and the objections cut toward the number as much as away from it. A 2025 review in JAAD Reviews found that no standardised comedogenicity test exists and that the label "non-comedogenic" is unregulated — a caution that applies to a borderline grade exactly as much as to a reassuring one.

Where you’ll see it

On a label it appears as Butyl Stearate. The one description of its use we can cite is the CIR's: the stearate esters it assessed are "primarily used in cosmetics as skin emollients at concentrations up to 25 percent," and the panel judged them safe as used at the levels it reviewed. That is a safety judgement, and the same report says comedogenicity is a separate effect that should be considered. We have not audited which product categories butyl stearate appears in today or how much of it they carry, and label position is not a dose, so we are not going to give you a list that reads like a market survey. The practical note is the neighbours. Do not read the butyl stearate number off a row for isobutyl stearate, a different branched-chain ester, or off the metal stearate soaps (magnesium, zinc, calcium stearate), which are not esters at all — Fulton 1989 scores zinc stearate 0 and magnesium stearate 1, and prints no row for calcium stearate. The grades above belong to butyl stearate specifically, CAS 123-95-5.

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.

1. 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, 1 mL to the inner ear of three New Zealand albino rabbits, five days a week for two weeks, follicular keratosis scored under a micrometer — three animals, a rabbit ear, not a human face. Its Table I prints butyl stearate at a comedogenicity grade of 3 on a 0-to-5 scale, with no asterisk, and outside the asterisked Oils section. Fulton's own key: 0 to 1 "not considered significant," 2 to 3 "borderline," 4 to 5 "considered positive," so the 3 is a borderline reading. Fulton called his survey "not at all definitive but simply designed to stimulate research." CosDNA and Face Reality print the same number, and acne.org now describes its ratings as a synthesis of multiple studies rather than a straight republication of this one; we have not traced who took what from whom, so we say only that matching numbers on consumer charts are not independent measurements.
2. Fulton JE, Pay SR, Fulton JE 3rd. "Comedogenicity of current therapeutic products, cosmetics, and ingredients in the rabbit ear." J Am Acad Dermatol 10(1):96-105 (1984) An earlier rabbit-ear survey by the same lead author, read in full off the journal scan. Method: two New Zealand albino rabbits per test, one ear dosed daily for two weeks and the alternate ear the control, borderline cases confirmed by microscopic biopsy. Its Table I prints butyl stearate at 4, and the row is unstarred — the paper's footnote says unstarred liquids and soft solids were used "as is," while starred entries were diluted with mineral oil or taken to 10% in an alcohol gel. Its verdict key DIFFERS from the 1989 paper's: "a minimal grade of 1 to 2 is not considered significant," "a grading score of 3 or above is uniformly reproducible and considered positive." Its abstract also names butyl stearate among offenders "such as isopropyl myristate and its analogs" — a classification, not a second measurement. Cited as a second Fulton reading that runs a grade higher than the 1989 one, not as independent corroboration of it.
3. Nguyen SH, Dang TP, Maibach HI. "Comedogenicity in rabbit: some cosmetic ingredients/vehicles." Cutan Ocul Toxicol 26(4):287-292 (2007) A later rabbit assay by authors unconnected to Fulton, and the only non-Fulton record of this material we found. Its abstract states that "the rabbit external ear canal was used to define which chemicals caused comedone formation on topical application," lists fourteen materials with butyl stearate third, and says "the first nine were deemed positive." WE HAVE READ THE ABSTRACT, NOT THE PAPER: the numerical score, concentration, vehicle, application schedule and animal count are not in it, and its scale is 0 to 4 rather than Fulton's 0 to 5, so we carry no grade from it. Cited for the one thing the abstract does establish — an independent positive classification of the exact named material — and flagged so nobody, including us later, mistakes it for a row we have read. Obtaining the full text is an open job.
4. Cosmetic Ingredient Review. "Final Report on the Safety Assessment of Butyl Stearate, Cetyl Stearate, Isobutyl Stearate, Isocetyl Stearate, Isopropyl Stearate, Myristyl Stearate, and Octyl Stearate." J Am Coll Toxicol 4(5):107-146 (1985) An expert-panel safety assessment covering butyl stearate among seven stearate esters, cited for three narrow things. It establishes what the ingredient is used for (an emollient at concentrations up to 25%, a lipstick drag-reducer, a water-repellent in nail polish); it concludes the seven are "safe as cosmetic ingredients in the present practices of use" on toxicity, irritation, sensitisation and phototoxicity grounds; and its abstract states that "comedogenicity is a potential health effect that should be considered when the Stearate ingredients are used in cosmetic formulations." So it measures no comedones and is not evidence for or against a comedogenic rating — and it does not claim to be one. We read the published final report, not a draft.
5. "Comedogenicity in cosmeceuticals: a review of clinical relevance, regulatory gaps, and future directions." JAAD Reviews (2025) A recent peer-reviewed review of the whole field, covering literature from 1972 onward. Cited only for the field-wide caveat that no standardised comedogenicity test exists and that "non-comedogenic" is an unregulated label — the caution that governs every number on this page. It is a review, so it synthesises the legacy assays rather than adding a measurement, and we do not use it as a source for any butyl stearate score.

Others in the same family

Esters 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 esters as a family ›

• Caprylic/Capric Triglyceride Clear Low
• Cetyl Acetate Clogging Low
• Cetyl Palmitate Clear Low
• Decyl Oleate Clogging Low
• Isocetyl Stearate Caution Low
• Isopropyl Isostearate Clogging Low
• Isopropyl Linoleate Clogging Low
• Isopropyl Myristate Clogging Low

Last reviewed 2026-08-01 · How we decide

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