Isn’t that what the Russians used?
That's how they assassinated Alexander Litvinenko, a retired FSB officer who was talking out of turn (and working for MI6) in exile in London. They slipped a dose into his tea at Itsu, a London sushi restaurant. Investigators estimated he ingested about 10 micrograms of polonium, which is 200 times the median lethal dose.
There has also been speculation that Yassir Arafat was killed by polonium. However, his case wasn't investigated until about nine years after his death. The half-life of polonium is 138 days, so it would have mostly been gone after 24 half-lives.