I don't know how accurate this information is but @33 mins, he mentions about Folate and how unmetabolised Folic Acid may cause cancer.
howirecovered.com/understan...
Also, there is something called Active B12 therapy.
He seems to suggest high dose oral with combination of other vitamins and mineras are better than injection. I don't know if you all will agree on this but..... I'll leave this here for you to ponder.