The patch test before hair colour: why it is done
The sensitivity test is carried out by applying a small amount of the product behind the ear or in the crook of the elbow forty eight hours before colouring, so that an allergic reaction is detected before the whole scalp is exposed.
gelisted editorial team · Updated 15 August 2026
The short version
- The test is done 48 hours before application.
- European cosmetics rules require warnings on hair colourants.
- Hair colourants must not be used on eyebrows or eyelashes.
- A previous reaction to a black henna tattoo is a significant warning sign.
Most people will never have a problem. The reason for the test is that those who do can have a severe one, with no prior warning.
How it is done
A small quantity of the product is prepared exactly as it will be used and placed on a discreet area, behind the ear or in the crook of the elbow, and left to dry uncovered. It is checked at 48 hours. Any redness, itching, swelling or blistering at the site contraindicates application. An absence of reaction does not guarantee future tolerance, which is why many protocols repeat the test before each application.
| Moment | Action | What to assess |
|---|---|---|
| 48 hours before | apply to a discreet area | no reaction means proceed |
| At 24 hours | first check | itching or mild redness |
| At 48 hours | definitive check | swelling or blisters contraindicate |
| First ever colour | test required | |
| Change of brand or formula | repeat the test | |
| After a previous reaction | do not apply | medical assessment |
| Pregnancy | seek advice first | |
| Usual processing time | 30 to 45 minutes |
What the label says
European cosmetic product rules require warnings on hair colourants, among them that they can cause severe allergic reactions, that they must not be used to colour eyebrows or eyelashes, and that anyone who has previously reacted to a hair colourant or to a black henna temporary tattoo should not use them. Those warnings are not formalities: they describe the highest risk situations.
Signs during application
Intense itching, growing stinging or a sensation of heat that increases through the processing time. Any of those means rinsing immediately with plenty of water for 10 to 15 minutes rather than completing the time. If swelling of the eyelids or lips or difficulty breathing appears, call 112: those signs indicate a reaction that rinsing will not resolve.
Mild stinging that disappears within the first minutes is different from stinging that grows, and the difference is easy to perceive.
Eyebrows, lashes and black henna
Hair colourants must not be used on eyebrows or eyelashes: separate products exist for that area under their own rules. Black henna temporary tattoos deserve a mention of their own, because they commonly contain substances that sensitise strongly. Anyone who reacted to one of those beach tattoos should always mention it in the salon before any colour service.
Frequently asked
+When is the patch test done?
Forty eight hours before colouring, on a discreet area such as behind the ear or the crook of the elbow.
+Does it need repeating?
In many protocols yes, and always when changing brand or formula, since tolerance can change.
+What do I do if it stings during application?
Rinse immediately with plenty of water for 10 to 15 minutes without completing the processing time.
+Can I colour my eyebrows with hair colour?
No. The label prohibits it and separate products exist for that area.
+Why do they ask about henna tattoos?
Because black henna can sensitise strongly and a previous reaction raises the risk.
Sources
- 1Regulation (EC) 1223/2009 on cosmetic products, European Union (August 2026)
- 2Ministerio de Sanidad, Gobierno de España (August 2026)