Small children don't laugh when you say "however."

When used as synonyms, the difference is 'but' is informal and used in spoken English, 'however' is formal and used in written English. Punctuation is also different ('I'd like to, but I can't' and 'I'd like to; however, I can't'). There is no other real difference if they are used as synonyms.

When used as synonyms, the difference is 'but' is informal and used in spoken English, 'however' is formal and used in written English. Punctuation is also different ('I'd like to, but I can't' and 'I'd like to; however, I can't'). There is no other real difference if they are used as synonyms.

Conjunction (in this case) vs Adverb

""But" implies a major difference in the expected result, whereas "However" informs the reader/listener to pay attention to something that may only be a slight variation of what is expected. example, "Not this but that" makes sense... "It clears your blackheads "however" it can also cause skin rashes and other side effects"