Experts agree that tattooing over a mole could someday cover up a major health concern.

Never tattoo over a mole.

That’s one of many things tattoo artist Joseph Anderson learned early in his apprenticeship. Now that’s he’s been tattooing for more than two years at the Body Shop in Appleton, Wis., he inspects each canvas before picking up his tattoo machine.

“I look over the area pretty well to check for moles and scar tissue, obviously. I have to avoid moles, so sometimes I will consider that in the design of the tattoo so any major line work will avoid going through them,” Anderson said.

Tattooing over a mole makes it harder to detect skin damage, especially skin cancers like melanoma. One of the surest signs of skin cancer is when a mole becomes discolored or misshapen.

Early diagnosis of melanoma drastically increases a person’s chance of survival, especially among light-skinned people who have a greater risk of developing skin cancers.

Dr. Ariel Ostad, assistant clinical professor of dermatology at the New York University School of Medicine, said there’s no evidence that tattoos increase a person’s likelihood of getting skin cancer.

“It is never a good idea, however, to have a tattoo placed too close to or within a mole. Changes occurring in a mole—symmetry, border, color, size, shape, texture—are potentially key warning signs that the lesion may be evolving into a melanoma or another skin cancer, so make sure all moles are left completely visible, or it could delay detection,” he wrote on the Skin Cancer Foundation’s website.

A new case study from Germany appearing in the latest issue of JAMA Dermatologyhighlights the difficulties of identifying skin cancer beneath a layer of tattoo ink.

A 29-year-old man was having a large tattoo over his shoulder removed at the Department of Dermatology at the University of Heidelberg. After 47 laser treatments, doctors couldn’t continue until he agreed to have a problematic mole removed, which turned out to be malignant melanoma.

Researchers said that while there are only 16 documented cases of malignant melanoma developing beneath tattoos, any moles under ink should be surgically removed before undergoing laser treatments.

“For safety reasons, tattoos should never be placed on pigmented lesions,” the JAMA researchers wrote. “If they are, the tattoos should not undergo laser treatment.”