Subcutaneous

Subcutaneous tissue has a modest blood supply, so absorption from it is slower and steadier than from muscle. For compounds meant to produce a sustained level — not a spike — that is the point, not a limitation.

It is also the route that makes self-administration reasonable: a short, fine needle at a shallow angle into abdomen, thigh or the back of the arm. That is why insulin syringes are the standard tool here.

Rotation matters more on this route than people expect. Repeated injection into one site can produce local changes in the fat that alter how the next dose is absorbed, which quietly makes a consistent dose behave inconsistently.