Should men on adt take a 5ar-inhibitor  when treating prostate cancer?

No. If you're already on adt, your testosterone is close to zero. 5 ari's only block the conversion of testosterone to dh-testosterone. So it is not only not fda approved, it won't help.
Not at this time. While some phyiscians do prescribe a 5 Alpha reducatse inhibotr when putting a ptainte on adt for prostate cancer, there is currently no proven advantage. What is more important is that currecnt recommendations are for men on adt to have therapy done intermittently. You should disucss this with our urologist to make sure it is the correct apporach for your care.
Compicated. It is not to my knowledge fda approved for that use.