How to Patch Test Hair Colour Shampoo: Step-by-Step
by Wooly Team
Every hair colour product containing PPD — which is most of them — needs a patch test before you use it. Setting one up takes about two minutes. The 48 hours afterwards are the part that actually matters.
Why 48 hours specifically
A PPD reaction is a delayed allergic response. It does not announce itself while you are applying the product. It typically appears hours later, and it can take up to 48 hours to develop fully. That is the entire reason for the waiting period: a patch you check after twenty minutes tells you almost nothing useful.
Wooly Hair Colour Shampoo specifies a patch test 48 hours before first use, and describes it as mandatory rather than recommended.
How to do it
- Pick the spot. The inner elbow or the skin behind the ear are the usual choices — thin skin, easy to keep an eye on, easy to leave alone.
- Apply a small amount of the product to a small area, the way you would actually use it.
- Leave it undisturbed. Do not wash it, cover it, bandage it or rub at it.
- Check at intervals across the full 48 hours — not just once at the end. A reaction that flares at hour 12 and settles is still a reaction.
- If the site is completely clear after 48 hours, you can go ahead with a full application.
What you're checking for
Redness, itching, burning, swelling, blistering, or a rash spreading beyond the patch itself. If any of these appear, do not proceed with a full application. Wash the area, and do not use the product.
Different products, different windows
The waiting period is set by the product, not by a universal rule, so read the pack you actually bought. Wooly Hair Colour Shampoo, which contains PPD and Resorcinol, specifies 48 hours. Wooly Beard Touch-Up is a different kind of product — a pigment stick with no PPD or Resorcinol — and specifies a 24-hour patch test. Same principle, different window, because the ingredient risk is different.
If you've reacted to hair colour before
A known hair-dye reaction is not something a patch test overrides. Wooly Hair Colour Shampoo is not recommended for sensitive or allergy-prone skin, and that guidance stands whatever a test shows.
It is also worth knowing that sensitisation develops with exposure. You can use a colour product uneventfully for years and then react to it. That is why the advice is to test each new product you buy, rather than treating the first test you ever did as a permanent clearance.
Bottom line
Two minutes of work and 48 hours of waiting, and you find out before you are committed instead of after. If you want to understand the ingredient that makes the test necessary, we have written separately about what PPD actually is and why it's in hair dye.
Wooly Hair Colour Shampoo requires a patch test 48 hours before first use, and the directions are printed on the pack. See the full details →