Long known as a poison and a carcinogen, arsenic also kills cancer cells, particularly in leukemia. Researchers now have evidence that arsenic's positive and negative effects stem from the same action, indirect inhibition of telomerase.
Telomerase keeps cells alive by repairing frayed ends of chromosomes. Without the enzyme, chromosomes fuse end-to-end, leading to genetic instability. In healthy cells, such instability can cause cancer. Conversely, in cancer cells, it can lead to cell death.
Vastag B. Arsenic Inhibits Telomerase. JAMA. 2001;286(23):2934. doi:10.1001/jama.286.23.2934-JHA10013-4-1