I am sorry for upsetting you that was not my intention, I am a vet tech with over 10 years experience, I have worked at the Bronx zoo’s animal hospital and I have a M.S in zoology. The exotics formulary that I have has the dose listed at 25 mg/kg for ball pythons specifically to be given every 14 days and not to be repeated more than twice due to neurological side effects. My concern is for your pets, you have no idea how many pets I have seen that where self medicated by owners who had the best intentions that have died. I also know this dose is correct because I recently had to give my own BP metronidizole and this dose worked perfectly. If the metronidizole that you have had is in powder form it is most likely crushed tablets and needs to be compounded it is not manufactured in a powder form, most vets will actually administer it orally from the I.V solution that is 5 mg/ml. If your vet is giving you metronidizole at 100 mg/kg please seek a new one that dose is absolutely wrong.
Once again I apologize for upsetting you; it just really upsets me when people take it upon themselves to medicate their pets. There are a number of issues that a reptile can have that present like parasites and there is no way to know for certain what is wrong with out diagnostic testing. It is also illegal for your vet to prescribe medication with out seeing your pet. It would be like you doctor giving you antibiotics with out diagnosing a problem in the first place, If they did that and you had a reaction to that medicine they would be talking to their malpractice attorney.
My info comes from Exotic Animal Formulary by James W. Carpenter M.S. DVM Dipl ACZM