The word however is an adverb and an adverb alone. The current online Oxford (Oxford English Dictionary Online, n.d.) and Cambridge (Cambridge English Dictionary Online, n.d. a) English dictionaries both have it listed solely as an adverb for British English. At the risk of awakening yet another descriptivist versus prescriptivist war, it must however be acknowledged that however is often used as a conjunction. This can, and frequently does, lead to confusion though, as the reader has to read on before realising that in fact that however was actually being used as a conjunction (or ‘connective’ in modern grammatical parlance). ‘However the cat walked down the street …’ surely has the reader thinking something along the lines of ‘In whatever manner the cat walked down the street …’ But a typical case of what I shall in this article call conjunctive howeveritis would reveal a complete (well, incomplete actually) sentence along the lines of, ‘However the cat walked down the street, even though it rarely ventured from the house.’ Not only are we now left with a sentence fragment, but in such an instance the reader would have to backtrack and subsequently assume the However was in fact being used as a coordinating conjunction. To me this is inefficient and an enemy of lucid writing. The Cambridge English Dictionary raises the warning flag high with the following example (Cambridge English Dictionary Online, n.d. b):

Warning:
We can't use however as a conjunction instead of but to connect words and phrases:
My teacher is very nice but a bit strict.
Not:
My teacher is very nice however a bit strict.