Allergy patch testing is important to that for every customer to do, even though henna a natural product, but also for the other products. Testing we do nowadays with every beauty product to see if allergic for a customer there is. Do we test on the wrist or behind the ears, after 24 h, there must be a response if necessary. Always there may be a very small percent that the Patch test is not done properly and still an allergy can arise, please inform the customer therefore there good at.
Even though we will think that Henna is a natural product, it is allergy free.
That is unfortunately not so, some people may also be allergic to vegetable products or dyes.
Hence it is necessary to always have a patch test.
Always use an informed consent for your client.
This is to specialist to protect from allergies or other conditions. And please do a PATCH TEST.