I’m not surprised — tattoo inks are not regulated and the colors are often formed with toxic compounds (e.g. copper). Then the needle avoids the protective barrier provided by the skin.

Ironically, while working on a drug program that involved intradermal injection, we needed to tattoo the injection site so we could find it later. The FDA was very demanding that both the procedure and more importantly to them the dye would not have any effect that might alter the body’s response to the compound under test. We had to do a whole study just to validate this.

So the FDA is concerned about this issue for guinea pigs, but is barred from investigating the effect on humans. All they can do is publish advice on their web site.