I am fully aware of the halacha which prohibits getting a tattoo. However, if someone who is a T1 insulin-dependent diabetic (which means that they need insulin to survive) wanted to get a medic-alert style tattoo on their wrist, for example, does the concept of pikuach nefesh override the prohibition? A medic-alert ID is usually worn by diabetics and others so that in the case of an emergency, medical staff or first responders will know that the person needs specific care and to prevent dangerous and life threatening emergency treatments.
Consider this. Doctors and soldiers are allowed to violate shabbat in order to go to work even if the immediate requirement of saving a life is not applicable or even in the case where theoretically it would be possible NOT to violate shabbat (driving or operating electrical equipment when other more cumbersome options are available). In this case, the person is violating the prohibition against getting a tattoo because there is a real need to protect their life even though, in theory, they could wear a medic-alert bracelet because a bracelet or other substitute can be lost or removed from their body.