Henna is a plant. When its leaves are crushed into a powder, it can be mixed with water or an acidic medium (such as lemon juice) to create a paste that will temporarily dye the skin. Essential oils are usually added to darken the henna stain.
When Henna Heals was operational (2010-2015), it was required that artists only use organic henna powder, water or lemon juice, and if using essential oils, to only use a high-altitude lavender oil with a camphor rate of less than 1% (camphor is toxic, but in Canada there is a legal limit of 11%). Lavender (not lavendine) is usually considered the most gentle oil.
It is recommended that you do a test patch 48 hours in advance of receiving a henna design to ensure you do not react to any ingredients.
Pre-made henna cones, or worse, “black henna” should NEVER be used. Pre-made cones can contain chemicals and preservatives that could be dangerous. “Black henna” can cause burning and permanent scarring. Black henna is illegal in Canada but is still used here and in many parts of the world.
In the United States, the Food and Drug Administration lumps natural henna and toxic black henna in the same category, so natural henna is only approved as a hair dye (as of 2015), not for use on the skin.
Natural henna has been used as a form of body adornment for thousands of years across Asia, Africa and the Middle East, but there isn’t much academic literature available: Henna is a plant, so it cannot be patented, meaning there is a lack of research funding.
That being said, here are a few medical journals that discuss the safety of henna, including its use as a temporary alternative to permanent tattooing for people undergoing radiotherapy. You are welcome to read these documents and share them with your medical team: