The death of former KGB agent Alexander Litvinenko by polonium-210 is still damaging relations between the UK and Russia. The evidence suggests that the polonium was added to a cup of tea served in the bar of the Millennium Hotel. Later, after it had been washed, the pianist drank out of the same cup and also showed evidence of polonium ingestion.
Polonium is a metal, so in order to be used as a poison it would have been administered as a soluble compound similar in form to table salt. To be fatal, all that would be needed would be about ten millionths of a gram, a speck almost too small to be seen with the naked eye.