I get a little thrill whenever I'm trying to shoehorn a clever rhyme into prose or a lyric — that little brain-tickle when a line snaps into place. When you ask which poison synonym rhymes with 'poison', the honest poetic pick I'd reach for is 'noisome'. It's not a perfect, ear-for-ear rhyme, but it's a near rhyme that actually shares meaning territory: 'noisome' can mean harmful, foul, or offensive — the sort of adjective you'd use to describe a thing that metaphorically (or literally) poisons an atmosphere. Phonetically, both words carry that NOY sound at the start, so in most spoken-word or stylized readings they sit nicely together.
If you want to be picky — and sometimes I am, when I'm editing fanfic or polishing a verse — 'noisome' ends with an /-səm/ while 'poison' ends with /-zən/, so it's technically a slant rhyme. But slant rhymes are my secret weapon; they let you keep accurate meaning without forcing awkward phrasing. Other direct synonyms like 'venom', 'toxin', or 'bane' don't match the 'poi-/noi-' vowel sound, so they feel jarringly different if you're after that sonic echo. One trick I use is pairing 'poison' with a two-word rhyme or internal rhyme — for example, "poison in the basin" or "poison sits like poison" — which lets you play with rhythm instead of chasing a perfect single-word twin.
If your wordplay is playful, go bold: try lines like "a noisome whisper, a poison grin" or "the noisome truth, like poison, spreads". If you need a tighter rhyme scheme, consider reworking the line so the rhyme falls on something that does rhyme (e.g., rhyme 'poison' with a phrase that sounds similar: 'voice on' or 'choice on' can be fun if you lean into slanting the pronunciation for effect). Bottom line — 'noisome' is my pick for a synonym that rhymes well enough to be satisfying in creative writing, and if you want I can cook up a handful of couplets using it in different moods.