Hi there.

I've had IV iron, vitamin D IM (intramuscularly), B12, B1, B6 IM but it's always been for severe deficiencies, not fixable with oral supplements during malabsorption...

I think if you don't have any malabsorption problems you can take b vitamins orally and they're just as effective as their injectable counterparts. I don't know about the vitamin C, but I'd definitely measure a vitamin D level before taking megadoses of it if I were you. Vitamin D is pretty safe when taken orally, at doses below 10.000 IU a day, but injectable D can go up to 600.000 IU per vial and this can really mess with one's calcium balance, especially without a deficiency and without specialist supervision....

Ziggy