The major differences between tattoo and pen inks are the composition of the inks and the purposes for which they are used. Tattoo ink is applied beneath the skin in a cosmetic procedure while pen ink is used on paper and other inorganic surfaces.

Traditional writing inks used by the Chinese 5,000 years ago were mixtures of soot suspended in water and a stabilizing agent such as gelatin or egg albumen. Later mixtures substituted berries and barks for the soot.

Tattoo ink was comprised of pigments extracted from plants, trees and flowers. Some early tattoo tribe cultures, such as the Maori, used burnt gums or caterpillars.

Modern inks are complex and their formulas protected by patents. They are most often comprised of dyes combined with various chemicals, including preservatives and thickeners that ensure smooth running ink that won't dry in the shaft of the pen. The coloring agents used can be organic and inorganic. Ball point pen ink is alcohol-based, although the specific ingredients are trade secrets.

Modern tattoo ink contains pigment less prone to fading, such as heavy metals, salts and other chemicals suspended in a carrier fluid such as ethyl alcohol or water. According to the U.S. Food and Drug Administration, the pigments used in tattoo ink are not regulated for injection into the skin. This absence of regulation allows tattoo artists to create their own mixes.

Pen manufacturers favor dyes as the ink colorant to prevent the pen from clogging. Tattoo ink uses dyes or pigments. Pen inks are standardized formulas. Tattoo inks are subject to customization by tattoo artists.

The use of tattoo ink and pen ink involves varying degrees of risk. Pen ink has not been proven hazardous when used as directed. The danger presented in tattoo inks is that not all of the ingredients used have been identified and inks are created in custom mixes, making it difficult to identify them all. Allergic reactions such as acute inflammation are possible.