Make sure to perform a patch test 48 hours before use, even if your client previously used coloring products. The purpose of the patch test is to make sure there are no allergic reactions to the henna.
To do this, prepare the henna and apply a small dot on the inside of the elbow. After 20 minutes, wash off with lukewarm water. If any reaction occurs during the processing time, rinse immediately and don’t use this product. If no reaction occurs within 48 hours, then usage is safe.
The allergy test, however, cannot predict future allergic reactions. Please consult a doctor if you have any doubts.