Tannopin is a non-existent word, yet its spelling can be explained using the International Phonetic Alphabet (IPA). "Tannopin" could be transcribed as /tænəʊpɪn/ in IPA. This transcription indicates that the first syllable has a short "a" sound followed by an "n" sound, while the second syllable has a long "o" sound and a "p" sound, and the final syllable has an "i" sound and an "n" sound. However, because "tannopin" has no meaning, it is not a standard word, and therefore, its spelling can vary.
Hexamethylene tetramine tannin.
A practical medical dictionary. By Stedman, Thomas Lathrop. Published 1920.