How to patch test a new skincare product before your whole face

A patch test tells you whether a new product suits your skin before it goes on your whole face. It matters most for products with an active ingredient, such as salicylic acid, benzoyl peroxide, adapalene, retinol or vitamin C. This guide follows the method the American Academy of Dermatology (AAD) publishes, and adds what the FDA allows acne product labels to say.

The AAD method, step by step

1. Pick the spot. Choose a quarter-sized area where the product will stay in place, such as the underside of your arm or the bend of your elbow. Use skin that is healthy, with no cuts, sunburn or rash.
2. Use the normal amount. Apply the same amount and thickness you would use every day. A larger amount does not make the test more accurate.
3. Leave it on as you normally would. A cream, gel or sunscreen stays on. A cleanser or mask that you rinse off stays on for five minutes, or as long as its instructions say.
4. Repeat twice a day for seven to 10 days. Some reactions only appear after the skin has met a product several times, so one application is too short a test.
5. Read the result. No red, itchy or swollen skin after seven to 10 days means the AAD's answer is to go ahead and use the product.

Keep other new products off the test spot for the whole test. If you test two things on one patch and it reacts, you will not know which one caused it.

The 3 day test on acne product labels

The FDA rules for over-the-counter acne products (21 CFR 333.350) allow a label direction called a "sensitivity test for a new user". It says to apply the product sparingly to one or two small areas with acne during the first 3 days. If no discomfort occurs, you follow the rest of the label.

This test uses the skin you plan to treat, so it shows how your face handles the product. It is optional text, so some labels print it and others leave it out. You can do both tests: the AAD arm test first, then the 3 day test on a few spots on your face.

What a reaction looks like

The AAD lists three signs: red skin, itchy skin and swollen skin. If you see any of them, gently wash the product off as soon as possible and do not use it again. A cool compress or petroleum jelly can calm the spot.

The Drug Facts label for Differin adapalene gel names the signs of an allergic reaction: itching, a rash, hives, and swelling of the lips or eyelids. The label says to stop use and ask a doctor if any of these appear, or if irritation becomes severe. The NHS says to call emergency services if your lips, mouth, throat or tongue suddenly swell, or if you are struggling to breathe. In the US, that number is 911.

Retinol and acids can sting during the test

The AAD says that some ingredients, such as retinol and glycolic acid, can irritate skin, most of all sensitive skin, and calls this irritation normal and temporary. The Differin label says the same about adapalene: redness, itching, dryness and burning are more likely in the first few weeks, and irritation usually lessens with continued use.

So a retinoid test spot that turns dry or pink during the first days can still pass. Hives, a rash, swelling, or irritation that keeps getting worse are the signs to stop. If you cannot tell which one you are seeing, stop the product and ask a pharmacist or doctor. The adapalene label expects some irritation in the first weeks, and how to start adapalene covers a slower start that makes it easier to handle.

One new product at a time

The AAD's advice for acne treatment is to use one product for 6 to 8 weeks and add no other product during that time. Patch testing follows the same idea: test one product, read the result, then move to the next. If you add two products in one week and your face reacts, there is no way to know which one to blame.

You do not need to test a product again if your face has used it for weeks without trouble. Test again when the formula changes or the strength goes up, for example from 2.5 percent to 5 percent benzoyl peroxide.

Skinli's beginner acne routine guide covers how to add a first acne treatment slowly, and what order to apply your acne products shows where a new product goes once it passes. If a treatment that passed the test seems to cause new pimples in the first weeks, skin purging vs a real breakout explains how to tell the two apart.

A patch test is one step of a larger plan. The complete acne skincare routine shows where each new product fits, morning and night.