Background
Research evidence suggests that blood thinners (called anticoagulants) may not only prevent life-threatening blood clots, but also have a direct anticancer effect. However, blood thinners can also increase the risk of serious and fatal bleeding. Therefore, it is important to understand the pros and cons of treatment to allow people and their doctors to be aware of the balance of risks and benefits.

Study characteristics
We searched the scientific literature for studies of anticoagulants in people with cancer. The evidence is current to December 2017.

Key results
We included seven trials including 1486 participants with cancer. Most trials included participants with various types of cancer especially lung and pancreatic cancer. We found six studies using warfarin and one study using apixaban. When considering people with cancer in general, warfarin had no effect on mortality (death rate) or the risk of blood clots. However, it increased the risk of major bleeding in 107 more people per 1000 population and minor bleeding in 167 more people per 1000 population. Apixaban had no effect on mortality, recurrence of blood clots in blood vessels, major bleeding or minor bleeding; however, these findings were based on only one study.

Certainty of the evidence
When comparing warfarin to no warfarin, we judged the certainty of the evidence (how sure and confident we are of the findings) to be moderate for mortality at one year and major and minor bleeding, low for symptomatic deep vein thrombosis (blood clot within a deep vein, most commonly the legs) and very low for pulmonary embolism (blood clot in the blood vessels of the lungs).

When comparing apixaban to no apixaban, we judged the certainty of evidence to be low for mortality at three months, major and minor bleeding, pulmonary embolism and symptomatic deep vein thrombosis.

Editorial note: this is a living systematic review. Living systematic reviews offer a new approach to review updating in which the review is continually updated, incorporating relevant new evidence as it becomes available. Please refer to the Cochrane Database of Systematic Reviews for the current status of this review.