The risk of severe bleeding is small but definite (a typical yearly rate of 1–3% has been reported), and any benefit needs to outweigh this risk when warfarin is considered. All types of bleeding occur more commonly, but the most severe ones are those involving the brain (intracerebral hemorrhage/hemorrhagic stroke) and the spinal cord. Risk of bleeding is increased if the INR is out of range (due to accidental or deliberate overdose or due to interactions). This risk increases greatly once the INR exceeds 4.5.Several risk scores exist to predict bleeding in people using warfarin and similar anticoagulants. A commonly used score (HAS-BLED) includes known predictors of warfarin-related bleeding: uncontrolled high blood pressure (H), abnormal kidney function (A), previous stroke (S), known previous bleeding condition (B), previous labile INR when on anticoagulation (L), elderly as defined by age over 65 (E), and drugs associated with bleeding (e.g., aspirin) or alcohol misuse (D). While their use is recommended in clinical practice guidelines, they are only moderately effective in predicting bleeding risk and do not perform well in predicting hemorrhagic stroke. Bleeding risk may be increased in people on hemodialysis. Another score used to assess bleeding risk on anticoagulation, specifically Warfarin or Coumadin, is the ATRIA score, which uses a weighted additive scale of clinical findings to determine bleeding risk stratification. The risks of bleeding are increased further when warfarin is combined with antiplatelet drugs such as clopidogrel, aspirin, or nonsteroidal anti-inflammatory drugs.
This has arisen independently and become fixed several times around the world. Other 4-hydroxycoumarins used as rodenticides include coumatetralyl and brodifacoum, which is sometimes referred to as "super-warfarin", because it is more potent, longer-acting, and effective even in rat and mouse populations that are resistant to warfarin. Unlike warfarin, which is readily excreted, newer anticoagulant poisons also accumulate in the liver and kidneys after ingestion. However, such rodenticides may also accumulate in birds of prey and other animals that eat the poisoned rodents or baits. Vampire bats
Warfarin is used to cull vampire bat populations in areas where human–wildlife conflict is a concern. Vampire bats are captured with mist nets and coated with a combination of petroleum jelly and warfarin. The bat returns to its roost and other members of the roost become poisoned as well by ingesting the warfarin after reciprocal grooming. Suspected vampire bat roosts may also be coated in the warfarin solution, though this kills other bat species and remains in the environment for years. The efficacy of killing vampire bats to reduce rabies transmission is questionable; a study in Peru showed that culling programs did not lead to lower transmission rates of rabies to livestock and humans. Occupational safety
People can be exposed to warfarin in the workplace by breathing it in, swallowing it, skin absorption, and eye contact. The Occupational Safety and Health Administration has set the legal limit (permissible exposure limit) for warfarin exposure in the workplace as 0.1 mg/m3 over an 8-hour workday.