This is from the Ancestral-Nutrition.com Web site: "It is true that some red inks used for permanent tattoos contain mercury, while other reds may contain different heavy metals like cadmium or iron oxide. These metals—which give the tattoo its “permanence” in skin—have been known to cause allergic reactions, eczema and scarring and can also cause sensitivity to mercury from other sources like dental fillings or consuming some fish. While red causes the most problems, most other colors of standard tattoo ink are also derived from heavy metals (including lead, antimony, beryllium, chromium, cobalt nickel and arsenic) and can cause skin reactions in some people."
The other consideration is the needles' acupuncture effect on the body, triggering different meridians.
For more details, please check the link: