Thank you for reading my question.
There is hey fever medications called Allegra. If I would say; I have to go get Allegra today. Do you native speakers use an article or some or a box of? Could you tell me the natural way of saying this?

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*hay fever
You can use it without an article -- it means an undefined amount of Allegra. It is acceptable in this case because it's a mass noun.

In certain contexts, you could say "an Allegra" -- it could refer to one unit of it, whether a single pill or a single package. That should be understandable from the context. E.g. "I'll go take an Allegra" = "take one pill".

You can say "some Allegra". This would also refer to an undefined amount, but "some" makes it seem a bit smaller than in the first example (that's my subjective impression though).

"The Allegra" would be entirely context dependent. E.g. "Could you go get Allegra for me? The one on the table, not the Allegra in the medicine cabinet."