People with tattoos can donate blood if their tattoo was applied  using sterile needles and ink that has not been reused. Those who have not received tattoos in above manner must wait 12 months after receiving the tattoo to donate blood. About 20 years ago and further, tattoo ink used sometimes comprised of small fragments of metal as well as other ingredients.  Some MRI patients who have had tattoos that dated back far enough to have received ink that contained metal bits have reported slight discomfort to severe pain during an MRI scan or blurring of MRI images.