Slate reports that d-Con rodent poison works by inducing fatal hemorrhaging in the animal that consumes it. However, the poison does not kill mice immediately. Instead, the mice must consume the poison several times to ingest a lethal dose. The poison is designed in this manner to prevent the mice from associating the bait with the death of other mice.
The active ingredient in d-Con is a chemical called brodifacoum. Slate explains that previous versions of the poison used a different but related active ingredient called warfarin. Warfarin was initially discovered in a patch of fouled clover hay, which killed cows that ate the food. Unlike brodifacoum, warfarin kills much more quickly.
One important aspect of brodifacoum is that it creates an insatiable thirst in mice and rats, as explained by Slate. This often leads the mice to venture outside after eating the bait, in search of water. This reduces the number of rodents that die inside the walls and crawlspaces of homes.
D-con is toxic to humans and pets as well as rodents. However, with prompt treatment, Slate reports that recovery is usually possible. Vitamin K1 is usually the treatment offered to humans who have ingested the poison, but in some cases, blood transfusions are necessary.