We have it in powdered form. For a snake that regurged: wait 2-3 weeks after the regurge, then use a very small prey item (for a baby that would take a rat fuzzy, use a large mouse pink or small mouse fuzzy) and dust it with the pro-biotic. The idea is to give the gut flora something to feed and reproduce on and not nourish the snake so much with this feeding. The following meal, 7 days later, offer something a little bigger than the mouse you dusted and work your way up again to the normal sized prey item. This gives the flora a chance to reproduce on it's own to accommodate the prey. The subsequent meals should not need dusting at this point.
Double check your temps and try to figure out why it regurged in the first place. A fecal test might be in order as well.
And just for fun... for an animal that was eating then decides to just stop for no apparent reason: If the fast goes on for a while and there is no health issues causing the snake to not eat, I'll mix some of the pro-biotic with the drinking water. About a week later I offer a very small prey item. This method has worked for me in getting some snakes eating again.
Good luck!