You may have not realised that subcutaneous injection and IV are two very different forms of 'into the body' injection methods.
Subcutaneous means under the skin, but not into the muscle.
IM is Intramuscular. IV is right into the vein.
IV is not something anyone in the general public is trained well enough to cope with performing even if they have their doctors support for serious recognised chronic medical conditions.
IV always involves entering a hospital to have the IV set up. IN some cases of long term chronic serious illness a portacath is set up so the patient can go home and inject into the portacath daily after training.
This is usually only done in serious cases such as end stage diseases.