I know the church doesn’t approve of tattoos, but what about one that is done for medical reasons?
The church understands and accepts medical necessity as a valid reason to do things that otherwise would not normally be a good thing to do.
Medical tattooing is done for one of two general reasons. One is to attempt to restore the body back to the way it was after a surgical operation, as much as it is possible to do so. The other is to aid in speedy and correct treatment of the patent. This can be anything from alignment markers to make sure radiation treatments get done in exactly the same spot or to warning of allergenic reactions or chronic disease.
In cases such as these the church’s position is to for you do discuss your options (because there are other options) with your doctor. Then with the guidance of prayer and your doctor’s expertise, make the choice that is best for your individual case and circumstance.