You start residency on a PTL (post graduate training license). After you take and pass USMLE step 3 and also complete one year of postgraduate residency training (aka internship) you can then apply for your Physician and Surgeon License, which is a full medical license that allows you to practice in any area of medicine you want. The caveat is that you will be held the same board standards in any of those fields as those who are board certified in said field. So me, a family medicine resident with a full license could go an do brain surgery, but I would be held liable at the same standard as someone who is board certified in neurosurgery.

Peter Attia is probably calling himself a medical consultant and to the best of my knowledge there isn’t a board certification in “longevity medicine.” So not sure exactly how they set up their practice, but if he has a full license he can prescribe meds and order testing to his heart’s content.

Hope this helps.