Why do blood thinners cause you to bruise?

Blood thinners cause bruising because they affect blood clotting, explains NYU Langone Medical Center. Bruising occurs when blood flows out of a damaged blood vessel and gets trapped under the skin.

Blood thinners are used to prevent heart attacks, strokes and other problems related to blood clots, states the Agency for Healthcare Research and Quality. In addition to bruising, blood thinners may also cause heavy menstrual bleeding, nosebleeds and prolonged bleeding from cuts. People who take blood thinners should be very careful when using sharp objects. They may also wish to avoid contact sports, wear gloves while gardening and always wear shoes outdoors to reduce the risk of injury.