Stearic Acid

Stearic acid is one of the ingredients on this site whose circulating number actually belongs to it: Fulton's 1989 table prints a stearic acid row and scores it 2-3 on a 0 to 5 scale — a range its own author called borderline. A second table in the same paper reads the material again in other vehicles, at 0 and 2. Borderline is what this site calls Caution, and that is where the rating sits.

What it is

A saturated fatty acid, eighteen carbons long (C18), also called octadecanoic acid, molecular weight 284.48. It is a hard, waxy, white solid at room temperature. Formulators use it as a thickener and a co-emulsifier: it gives a lotion or cream its body, and where a formula also contains an alkali — sodium or potassium hydroxide, or the amine triethanolamine — the acid is neutralised in the pot into a stearate soap (sodium stearate, potassium stearate, TEA-stearate), and that soap is then the emulsifier or cleanser. Where that happens, the stearic acid named on a label is not the substance left in the tube. Whether it happens depends on the formula, and we hold no figures on how often it does. One point of identity matters before any number is quoted. "Stearic acid" of commerce is usually not the pure C18 compound but a mixture of stearic and palmitic (C16) acids, in proportions that vary by grade and supplier; we hold no composition figure for a named grade. Fulton's own score for this row carries an asterisk, and in his paper that asterisk means "results depend on source of raw material" — which is exactly what a variable stearic/palmitic blend would produce.

Why we rate it Caution

We rate it Caution, at Low confidence, and the rating follows the key printed in the paper the number comes from rather than any argument we have made about the molecule. On many pages of this site the circulating number turns out to belong to a different material, or to no measurement at all. Stearic acid is not one of those. James Fulton's 1989 rabbit-ear survey in the Journal of the Society of Cosmetic Chemists prints a "Stearic acid" row in Table I and scores it 2-3 for comedogenicity on a 0 to 5 scale, with an asterisk the paper defines as "results depend on source of raw material". We render the table at 400dpi and read all 220 rows by eye, because OCR drops the numeric columns, and this row says what it appears to say. The number people quote for stearic acid is a measurement of stearic acid, which on the evidence we hold is not something we can say about most ingredients here. The method behind it: the ingredient diluted to roughly 10 percent in propylene glycol, 1 ml applied to the inner ear of New Zealand albino rabbits, three rabbits per assay, five days a week for two weeks, follicular keratosis then measured with a micrometer. And Fulton's own key to his own scale, which almost nobody quotes beside the number: "a minimal grade of 0 to 1 is not considered significant. Grade 2 to 3 is borderline. However, a grade of 4 to 5 is uniformally reproduceable and considered positive." By the standard of the man who produced it, a 2-3 sits in the borderline band from one end to the other: below his threshold for a positive result, above the band he called insignificant, and reaching 3 at its top. Borderline is what this site calls Caution, and that is the whole of the reasoning behind the rating. It is not a clean bill of health, and it is not a positive finding either. The same paper reads the material a second time, in different vehicles, and this is the part the charts never carry. Fulton's Table III prints "Stearic acid" at 0 in an organic solvent — the table's footnote reads "Ethyl ether or acetone" — and at 2 in sunflower oil. That is same-paper vehicle evidence, not a second laboratory: the same investigator, the same model, the same survey, with the carrier changed. Taken with Table I's 2-3, what it says is that the figure moved with what the acid was carried in, and that the top of the printed range is a 3. An older rabbit experiment describes rather than scores. Kanaar 1971, in Dermatologica, applied a series of fatty acids to the external ear canal of albino rabbits for six weeks; as summarised in the Cosmetic Ingredient Review's fatty-acid report, stearic acid produced "no distinct alterations", at the far end of a range that ran up to defined erythema, desquamation and persistent follicular keratosis for oleic and lauric acids. That is a qualitative description, not a grade, and we are not going to convert it into one — it sits on no scale. Fulton's neighbouring fatty-acid rows are worth printing as rows and nothing more: caprylic 1, capric 2, lauric 4, myristic 3, palmitic 2, stearic 2-3, eicosanoic 2, behenic 0. Stearic acid is not lauric acid, and in the same table they carry different numbers. What we will not do is build a chain-length law out of that column and use it to argue the printed 2-3 down to Clear. The rating follows the paper's own key, and the key calls this range borderline. One qualifier the rating carries, and the reason the ratings file flags a conflict on this ingredient: the form matters. Fulton's table prints a separate row, "Stearic acid: TEA", at 3 — the top of the plain acid's own printed range, and not an isolated assay of the salt. His text reports the entry as a formulation experiment: "In testing different ratios [4:1, 1:1, 1:4] of stearic acid to triethanolamine (stearic acid:TEA) in a cold cream base, all combinations were found to be comedogenic." That is evidence about a finished cold-cream formula, not a clean reading of TEA-stearate; an earlier version of this page called it the salt scored on its own, and the emulsifier hub's external fact-check caught the error. Face Reality's consumer list flags the TEA-stearate form as pore-clogging. TEA-stearate is a different material from stearic acid — a soap made from it — no isolated comedogenic reading of it appears in the corpus we hold, and the number and the rating on this page are for the plain acid, not its salts.

What the evidence doesn’t tell you

Rabbit-ear readings are still rabbit-ear readings, and here they all come from one investigator. This is the same assay we discount everywhere else on this site, and the reasons do not stop applying because a particular number is convenient. Draelos and DiNardo argued in 2006 that rabbit-ear results do not reliably predict comedogenicity in human skin; the 2025 JAAD Reviews survey of the field found that no standardised comedogenicity test exists and that "non-comedogenic" is an unregulated claim. Those are the grounds on which we refuse to trust the 4s. They apply identically to a 2-3, to Table III's 0 and 2, and to a qualitative negative. Two other primary inventories are silent on this material, and one of them invites a misreading. Fulton's own earlier 1984 rabbit-ear table, which we hold in full, has no stearic acid row. Nguyen, Dang and Maibach's 2007 survey has none either; it does test stearyl alcohol and butyl stearate, which are a fatty alcohol and an ester rather than this acid, and neither reading transfers here. With Morris and Kwan also carrying no stearic acid row, Fulton 1989 is the only paper among the four we hold that puts a number on stearic acid. We identified no study that counted comedones on human skin after applying stearic acid. Stearic acid does turn up in the human acne literature in two other ways, and neither is a comedogenicity test of it: it appears in surveys of how often flagged-comedogenic ingredients occur on the labels of products acne patients use, and it appears in analyses of the fatty-acid make-up of sebum. Where an ingredient sits on an ingredient list is not a dose, and the composition of the oil a follicle already produces is not a measurement of what a topically applied ingredient does to it. We have not found either kind of study run on stearic acid in a way that would move this rating, and we are not going to borrow one to. The material identity is looser than the label suggests, in a way that cuts both ways. The stearic acid Fulton tested carried his "results depend on source of raw material" asterisk, and commercial stearic acid is a stearic/palmitic mixture whose proportions vary by grade and supplier. Fulton scored palmitic acid 2 and stearic 2-3, so a blend of the two lands in the same borderline band from either direction; but the point stands that the "stearic acid" in a jar and the "stearic acid" on a rabbit's ear in 1989 were not necessarily the same proportions of the same two acids. We will not argue the rating from molecular size. The familiar online defence of the heavier fatty acids and fatty alcohols — that the molecule is simply too large to enter a follicle — is a mechanism we invented on a neighbouring page and had to retract, because the arithmetic did not support it. Whatever Fulton's molecular-weight observation is worth, and it is an observation about his own ingredient set rather than a law, it is not doing any work here. The rating rests on what the assays found, not on a story about why they found it. Finally, the salt forms are a real and separate question this page does not answer. Fulton's "Stearic acid: TEA" entry — a cold-cream formulation result, not an isolated assay of the salt — reads 3, the top of the plain acid's printed range, and where a formula neutralises the acid it is a stearate soap (TEA-, sodium or potassium stearate) that the product carries. We rate the plain acid here; the salts are their own materials, and the evidence on this page does not transfer to them.

Where you’ll see it

Creams, lotions, cleansing bars and milks, and lip and colour products, where it does the mechanical work of thickening and holding an emulsion together. On a label it reads simply as Stearic Acid, often high on the list — but in a formula that neutralises it with an alkali or with triethanolamine, that position reflects the soap it becomes rather than the acid it started as. Those are illustrative categories, not a measured prevalence; we have not audited the market. Nearby you will meet its own salts — sodium, potassium and TEA-stearate — and its neighbours on the fatty-acid curve, palmitic and lauric acid. They are related materials, they carry different numbers, and the evidence on this page does not transfer to them.

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 for the 0 to 5 scale nearly every list online still uses, read off a 400dpi render of the journal scan rather than via a chart quoting it. Design — ingredient at roughly 10% in propylene glycol, 1 ml to the inner ear of New Zealand albino rabbits, three rabbits per assay, five days a week for two weeks, follicular keratosis measured with a micrometer. Its Table I prints "Stearic acid" at 2-3, with an asterisk the paper defines as "results depend on source of raw material", and prints the separate row "Stearic acid: TEA" at 3 — an entry the paper's text ties to ratios of the acid and triethanolamine tested in a cold cream base, a formulation result rather than an isolated reading of the salt. Its Table III reads the same material again in other vehicles — "Stearic acid" at 0 in an organic solvent (that table's footnote reads "Ethyl ether or acetone") and at 2 in sunflower oil — which is same-paper vehicle evidence and not a second laboratory. What it cannot support — Fulton calls the survey "not at all definitive but simply designed to stimulate research", warns the rabbit ear is so sensitive that "not everything that irritates this model will also irritate human skin", and defines his grades such that 2 to 3 is borderline rather than positive. One animal assay, one laboratory, at the assay's roughly 10% in propylene glycol — a concentration we have no measured in-use figure to set beside.
2. "Final Report on the Safety Assessment of Oleic Acid, Lauric Acid, Palmitic Acid, Myristic Acid, and Stearic Acid." J Am Coll Toxicol 6(3):321-401 (1987); with CIR's 2019 re-review, "Safety Assessment of Fatty Acids and Fatty Acid Salts as Used in Cosmetics" An industry-funded, independently panelled toxicological safety review — not a comedogenicity study. Cited for two things we could read in it: the detailed summary of Kanaar 1971 that this page relies on for the rabbit-ear result, and the characterisation of stearic acid as a commercial material (a C12-to-C18 fatty acid, in commerce a mixture rather than the pure compound). Its overall conclusion is that these fatty acids are safe as used; safety is not noncomedogenicity, and the report counts no comedones.
3. Kanaar P. "Follicular-keratogenic properties of fatty acids in the external ear canal of the rabbit." Dermatologica 142:14-22 (1971) The earliest primary rabbit-ear experiment on the fatty acids, eighteen years before Fulton. Cited at one remove and labelled as such: we were unable to obtain the full paper, and the method and stearic-acid result described here are as summarised in the CIR fatty-acid report above, which is not the same as having read it. What we draw from it is narrow — stearic acid sat at the no-change end of its range, where oleic and lauric acids sat at the keratosis end. It is qualitative, on no scale, and we do not give it a number.
4. Fulton JE, Pay SR, Fulton JE III. "Comedogenicity of current therapeutic products, cosmetics, and ingredients in the rabbit ear." J Am Acad Dermatol 10(1):96-105 (1984) Fulton's earlier rabbit-ear paper, on its own 0 to 5 scale, whose key reads 1 to 2 not significant and 3 or above considered positive. Cited here for what it does NOT contain: no stearic acid row. We hold its table in full, so the absence is verified rather than assumed, and it means the 1989 figure has no earlier reading of his own to be checked against.
5. Nguyen SH, Dang TP, Maibach HI. "Comedogenicity in rabbit: some cosmetic ingredients/vehicles." Cutan Ocul Toxicol 26(4):287-292 (2007) The only later rabbit-ear survey we could find, on a 0 to 4 scale, cited for what it does not contain: no stearic acid. It does test stearyl alcohol and butyl stearate — a fatty alcohol and an ester, distinct materials from this acid — and those readings are not borrowed here. The last of the four primary inventories we hold, and the last place a second number could have come from.
6. Morris WE, Kwan SC. "Use of the rabbit ear model in evaluating the comedogenic potential of cosmetic ingredients." J Soc Cosmet Chem 34:215-225 (1983) The second major rabbit-ear survey, on the same 0 to 5 scale, cited here for what it does NOT contain: its 32 rows cover lanolins, vegetable oils, esters and surfactants, and stearic acid is not among them. It is the paper that would most plausibly have corroborated or contradicted Fulton's figure, and it did neither. It matters for a second reason — it contradicts Fulton in print on other materials, so "one assay found 2-3" cannot be read as "the literature agrees".
7. Draelos ZD, DiNardo JC. "A re-evaluation of the comedogenicity concept." J Am Acad Dermatol 54(3):507-512 (2006) A "Current Issues and Opinion" piece testing finished products on a small number of upper-back panels — not a same-substance rabbit-versus-human validation study. It argues that rabbit-ear results do not reliably predict comedogenicity in human skin and that testing an isolated ingredient says little about a finished formula. Cited against our own rating as much as for it: it is why the confidence here stays Low on a borderline score and a qualitative negative.
8. "Comedogenicity in cosmeceuticals: a review of clinical relevance, regulatory gaps, and future directions." JAAD Reviews (2025) A recent peer-reviewed narrative review covering the literature from 1972 to 2025. Finds that no standardised comedogenicity test exists and that the "non-comedogenic" claim is unregulated. A synthesis, not new data, and not independent of the legacy assays in the sense of testing anything — but independent of the consumer lists that republish them.
9. Acne.org, "What Is Comedogenicity, and What Ingredients Are Comedogenic?" A commercial acne-products site that also publishes an ingredient list, cited to characterise the secondary literature accurately rather than lazily. It states a rule for its list and cites primary papers, but it is still a list compiled from other people's animal work, with no measurement of its own. Named, alongside CosDNA, Face Reality and Banish, so a reader knows where the number they have already seen came from; they are not independent corroboration of it.

Others in the same family

Fatty acids 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 fatty acids as a family ›

• Capric Acid Caution Low
• Lauric Acid Caution Low
• Myristic Acid Caution Low
• Palmitic Acid Caution Low

Last reviewed 2026-08-12 · How we decide

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