Before applying henna, always perform a patch test to avoid allergic reactions. Mix pure henna with water, apply a small amount to your inner wrist, and wait 24-48 hours. Watch for redness, swelling, itching, or blistering. Avoid "black henna" containing harmful PPD chemicals. Even if you've used henna safely before, test each time—sensitivities can develop. This simple 5-step protocol could save you from painful chemical burns and serious skin damage.