It depends on the person and their specific needs. I don't absorb from oral intake very well, so I have to take larger amounts of things to get enough for my body to function. I feel horrible when I don't take calcium and magnesium (muscle pain & cramps, obstipation, stressed out), my red blood cell counts drop when I'm not taking iron supplements, etc., because I cannot get enough from my diet alone (food restrictions due to allergies/intolerances, malabsorption).
I don't see a difference between injections and oral intake of supplements except in cases of malabsorption of oral nutrition.
Chronic Lyme Disease, Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication/Chemical Allergies & Intolerances, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD ("Secondary Lupus-Like Syndrome"), Osteoporosis, chronic Lymphopenia, intermittant pancytopenia, chronic malabsorption/malnutrition, etc.; G-Tube; Currently TPN-dependent.
Meds: Pulmicort, IV Ceftazidime, Heparin (to flush PICC line), Claritin, Domperidone, Colloidal Silver (used topically), probiotics, homeopathy.