Warfarin sodium, C19
is an anticoagulant
which acts by inhibiting vitamin K
Warfarin is used in various medications for treatment of conditions where blood clotting is undesirable, including many heart conditions, and stroke.
It can also be found as the main active ingredient of Ratsak, a rat poison. The Warfarin causes the rats to internally bleed to death.
A sufficient dose would have a similar effect on humans - it could potentially render a tiny bruise fatal.