Is however an adverb?

Yes, however can be classified as an adverb when used as follows:

As a conjunctive adverb to mean “but” (e.g., It is currently designated as agricultural land. However, this could change next year when new legislation is introduced.)
To introduce a dependent clause to mean “no matter how” (e.g., “It needs to be finished today however long it takes.”)
Like an interrogative adverb to express surprise in a question (e.g., “However could you possibly think that?”)

You may need to include a comma before or after “however” depending on how and where it is used in a sentence.

Commas: Other interesting questions

Do you use a comma or semicolon before however?: Both are possible when however is used as a conjunctive adverb to mean “but.”

We put a semicolon before however when it is showing contrast between two independent clauses (e.g., “We have not reached our goal; however, we are only at the beginning of our journey”).

We put a comma before however when it is used at the beginning or at the end of a sentence or clause (e.g. “We are, however, only at the beginning,” “We are only at the beginning, however”).