How to patch test a new product

Quick answer Apply the product to a quarter-sized spot on your inner forearm or the crook of your elbow twice a day for 7–10 days. If there's no redness, itching or swelling, you can start using it. If a reaction appears, wash it off and stop using the product.

A patch test means trying a new product on a small area of skin for several days before using it all over. It's the simplest way to spot a product that irritates your skin or triggers an allergy, before it affects your whole face.

Why a quick dab isn't enough

Skin can react in two main ways:

• Irritation happens when a product damages the skin's outer layer. It's the most common type of reaction and can build up with repeated use.
• Allergy happens when your immune system reacts to an ingredient, such as a fragrance, preservative or dye. It's less common.

Both can take hours or days to show up, so a single dab for a few minutes doesn't tell you much. Labels such as “hypoallergenic” or “for sensitive skin” have no official definition in the US, so they don't mean a product is safe for you.

How to patch test, step by step

1. Pick a spot where the product won't be rubbed or washed off: the inside of your forearm or the crook of your elbow.
2. Apply a normal amount to a quarter-sized area, about 1 inch (2.5 cm) across, as thick as you'd use it on your face or body.
3. Repeat twice a day for 7–10 days.
4. For wash-off products such as cleansers and masks, leave them on for about 5 minutes, or as long as the label says, then rinse.
5. Check the area every day for redness, itching, swelling, bumps or blisters.
6. No reaction after 7–10 days? You can start using the product.

An optional extra step for the face: after the arm test, use the product on a small patch of facial skin, such as near your jawline, for a few days before using it everywhere. Facial skin is thinner and can react even when your arm didn't.

Normal tingle or a real reaction?

Active ingredients such as retinol and glycolic acid often cause some irritation at first, which can be normal and temporary. That's different from an allergy. If you're unsure, stop and ask a dermatologist.

If you react

1. Gently wash the product off right away.
2. Stop using it.
3. A cool compress or a thin layer of petroleum jelly can soothe the area.
4. Keep the ingredient list, so you can spot repeat ingredients in other products.
5. If the reaction is severe or doesn't improve, see a dermatologist. They can do clinic patch testing, usually over 2–3 visits, to find the ingredient you react to.

Tips that make testing easier

• Test one new product at a time. If you start three at once and react, you won't know which one caused it.
• “Unscented” and botanical don't mean reaction-free. Plant extracts and essential oils can trigger allergies too, and unscented products can still contain fragrance ingredients.
• Allergies can appear later. You can react to a product you've used for years, and sensitivities can get worse over time.
• Hair dye comes with its own skin test instructions. Follow them every time, even with a shade you've used before.
• A patch test doesn't check pregnancy safety. If you're pregnant or breastfeeding, ask your doctor about retinoids and high-strength salicylic acid.

Before adding a product, see where it fits in your skin care layering order. New face masks and exfoliating acids are especially worth testing; see how often to exfoliate.