I have no direct knowledge, but consulting sources on the internet (just like the previous poster!) it appears the main difference is that dexamethasone is very much more potent.
I’ve got to be honest, and say that I wouldn’t have spotted this one, and would have assumed that if it’s prescribed by my neuro, it must be OK, as he’s prescribing MS stuff all the time…
In view of the comparative potency, it does seem a very high dose of this atypical prescription, though.
Maybe query with the GP, or with the pharmacist, if you don’t want to risk antagonising the neuro directly, by appearing to question his judgement. It may be correct, but I certainly wouldn’t just take them without querying, now that you have raised the doubt.
It may even be that the secretary has a standard template for letters of this type, and she has substituted in the drug name (it may be that there’s a reason he wants you to have this one), but not edited the default dosage? If so, it should have been spotted, but I think a lot of them go out without being checked by the neuro ,these days, to save time. Some of them even say as much, in a rider at the bottom.