Slang consists of a lexicon of non-standard words and phrases in a given language. Use of these words and phrases is typically associated with the subversion of a standard variety (such as Standard English) and is likely to be interpreted by listeners as implying particular attitudes on the part of the speaker.
One caveat with D-mannose is that it is only effective with UTIs caused by E. coli infection. While this accounts for about 90% of cases, there are 10% that will not benefit from this treatment. In this case, supplements that help disrupt biofilms can be useful in treating and preventing UTIs.
D-mannose is very safe, even for long-term use, although most women (or the very occasional man) with single episodes of bladder or urinary tract infection will only need it for a few days at most. Although D-mannose is a simple sugar, very little of it is metabolized.