Non-comedogenic means a substance or product has not been shown to cause the formation of comedos or acne (in tests, at least). Conversely, comedogenic substances can cause an acne-like allergic reaction and aggravate existing symptoms of acne vulgaris.
Comedogenic Products and Acne
Comedogenic products can cause an acne-like reaction and aggravate existing acne symptoms
Acne sufferers are strongly encouraged to avoid products that contain known comedogenic substances because these can worsen acne symptoms. Most products designed for use on acne prone skin do not include ingredients that are known to be comedogenic. However, some moisturizing creams, lotions and make-up products do include oil-based ingredients that can be very problematic for certain individuals.
It is generally recommended that acne sufferers use glycerol based moisturizers (e.g. Cetaphil) to avoid aggravating acne symptoms. Some substances (eg. coal tar) are highly comedogenic and produce a type of allergic, acne-like reaction. Not all substances are equally comedogenic for all individuals and you should keep track of how you personally react to specific things.
How is comedogenicity determined?
The inside of rabbit ears are used as a substitute for human skin for comedogenicity testing
The majority of comedogenicity testing is done using a rabbit ear model. In this test, the substance is applied to the inside of the rabbit’s ear, which has a similar structure as human facial skin. The development of comedos is quantified to evaluate the relative comedogenicity of a substance.
In some cases, human volunteers are used to evaluate substances. In these tests, the most common treatment area is the skin on the back.
Comedogenicity testing reports often use different metrics for determining comedo formation and the results from these studies are not necessarily consistent.
What kinds of things impact comedogenicity?
Comedogenicity is a complicated process that can vary from individual to individual. One interesting observation is that human sebum is itself comedogenic. A substance can be comedogenic for several reasons:
It can contribute directly to the formation of a plug in the follicle (pore). This could potentially occur with a substance that triggers the coagulation (increases the “stickiness”) of free sebum. Plugged pores make it easier for P. acnes bacteria to grow in the follicle, which leads to inflammation.
A substance can be comedogenic because it triggers an allergic reaction and/or inflammation. For example, substances like SLS (sodium lauryl sulfate) are common ingredients in topical preparations. However, SLS can cause allergic reactions in some individuals and is generally considered comedogenic.
A substance can serve as a direct food source for the bacteria responsible for most responsible for acne vulgaris, P. acnes. P. acnes bacteria eats fatty acids as its primary food source and certain substances like olive oil or other vegetable oils could potentially serve as food sources and encourage bacterial growth. The increased bacteria can in turn stimulate increased immune response, inflammation and comedogenecity.
Some substances that are normally non-comedogenic can be converted into allergens by the enzymes present in the skin, or even by UV light.
There is not a clear consensus when it comes to the comedogenicity of many substances. Some tests indicate that certain substances are highly comedogenic, while other tests produce the opposite results. Because of these discrepancies, there is not necessarily a definitive list of comedogenic substances. Individuals who suffer from acne need to monitor the products they use, and discontinue the use of products that they identify as having adverse effects.
Common Comedogenic Substances
There are several lists of comedogenic substances available in different places on the internet. However, a review of the scientific literature reveals a serious lack of actual testing of commonly used ingredients. This may be because many companies do their own testing and do not publish the results, but it also casts some doubt on some of these online comedogenic substances lists.
That said, the editors at The Science of Acne are currently working to compile a comprehensive list of comedogenic substances from published scientific journal articles. Until then, we have included this table from the original comprehensive comedogenicity testing done by Dr. Fulton, et al. In these lists, comedogenicity and irritancy are graded on a scale of 0 to 5, with 0 being no effect and 5 being highly comedogenic.
Comedogenicity of Common Substances List Part 1 (Fulton, et al)
Comedogenicity of Common Substances List Part 2 (Fulton, et al)
Comedogenicity of Common Substances List Part 3 (Fulton, et al)
Michalun. 2009. Milady’s Skin Care and Cosmetic Ingredients Dictionary
Draelos. 2008. Procedures in Cosmetic Dermatology Series: Cosmeceuticals with DVD, 2e
Habif. 2003. Clinical Dermatology: A Color Guide to Diagnosis and Therapy
Comedogenicity and Irritancy of Commonly Used Ingredients in Skin Care Products.
Fulton, et al. 1989. For article abstract, click here. A re-evaluation of the comedogenicity concept.
Draelos, et al. 2006. For article abstract, click here. Comedogenicity of Squalene Monohydroperoxide in the Skin after Topical Application.
Chiba, et al. 2000. For article abstract, click here. An Experimental Study on the Comedogenicity of Several External Contactants.
Ahn, et al. 1985. For article abstract, click here.