Oral, sublingual, as well as injectable MB12, AB12, and HB12 have been helpful In many patients, without Greg's oil.
There is nothing special Greg's B12 oils. B12 injections would be just as good.
But Greg's B12 oils are more convenient to administer (it literately takes 10 seconds to apply one dose from the pump bottle to your skin), and this B12 oil works out cheaper than injections (less than $1 for a dose of the oil, whereas an injection would be several dollars).
One dose of the oil is equivalent to about four 500 mcg B12 injections.
But we need to take any claims that major chronic diseases like ME/CFS are due to a vitamin deficiency with a pinch of salt.