Why is the 48-hour patch test mandatory and how do I perform it correctly?

Oxidative hair dyes contain precursors (such as paraphenylenediamine) that can sensitise the immune system over time. A reaction may not appear on the first, second, or tenth use, but can suddenly occur as a severe contact dermatitis — redness, swelling, itching, blistering — not only on the scalp but also on the face, neck, and eyelids. To test, mix a tiny amount of colour cream and developer in the same 1:1 ratio. Using a cotton bud, apply a 1 cm spot behind the ear or on the inner forearm. Leave uncovered for 48 hours; do not wash, cover, or apply other products on that spot. If any redness, burning, itching, swelling, or blistering appears — even mildly — do not use the product on your head. Wash the test area immediately and consult a pharmacist or doctor. Repeat this test before every single colouring session, even if you have used this exact shade before, because sensitivity can develop at any time. Also skip the test if the scalp is already irritated, sunburned, or has open cuts.