it is a probiotic, just like humans get when they eat yogurt with active cultures. So you can't really "overdose" (of course, even water can be toxic if you take in TOO much!). It is just a fine powder that contains 6 microbes that SHOULD already be in reptile guts. But sometimes the "good guys" get outnumbered by the "bad guys" in people or other animals because of illness, stress, toxins, medications, or any number of problems. All NB does is put large numbers of the good microbes back in to try to out compete the "bad" microbes. All animals have pathogenic microbes in their bodies, but in healthy animals, they are far outweighed by the beneficial ones.
After a 7-10 day fast after the first (or any) regurge, it is really important to feed a tiny little meal, no more than half the size of the one that caused the regurge. You wouldn't give Thanksgiving dinner to somebody just getting over the flu, lol!
You can use the NB in (injected) or on the pinks if the snake is not doing too badly. But after 2 regurges (and certainly after 3 or more), I would consider mixing as much of the powder as you can get into 1 - 2cc of water (for a baby corn) and using an eyedropper or feeding tube to put it right down his throat. Hold the snake upright (head up, tail down) for a minute or so to let it go downhill to his stomach, before replacing in cage. Repeat every 3 or 4 days for a couple of more times, and only then give a tiny meal. Then do the tubing again a couple of times and another tiny meal. If no more regurges, you might be able to give tiny meals rolled in NB for a month or so. Each snake is different, so there is no way to say EXACTLY what will work. But be VERY conservative about getting it back on a "normal" diet. You might think he is cured after a few meals without a regurge, but if you go back to the usual diet too quickly, you will get a stinky surprise, and then have to start the regurge protocol all over again.
If you didn't already do so, be sure to read the regurge protocol in the FAQ section.
Good luck!