Tattoo inks have been described as "remarkably nonreactive histologically". However, cases of allergic reactions to tattoo inks, particularly certain colors, have been medically documented. This is sometimes due to the presence of nickel in an ink pigment, which triggers a common metal allergy. Occasionally, when a blood vessel is punctured during the tattooing procedure, a bruise/hematoma may appear.
Mainstream art galleries hold exhibitions of both conventional and custom tattoo designs, such as Beyond Skin, at the Museum of Croydon. Copyrighted tattoo designs that are mass-produced and sent to tattoo artists are known as "flash", a notable instance of industrial design. Flash sheets are prominently displayed in many tattoo parlors for the purpose of providing both inspiration and ready-made tattoo images to customers.