Blood-thinning medications, anticoagulants
In theory, because the anthocyanidins in bilberry may inhibit blood from clotting, there may be an increased risk of bleeding in those taking anthocyanidin extracts from bilberry along with blood thinners, particularly warfarin. This has not been tested scientifically, but those taking warfarin or other blood thinners in the same class, known as anticoagulants, should be very careful if considering use of bilberry and should be followed closely by a doctor who will check your INR (a measurement to indicate, in part, your risk of bleeding).

There has been one case report of a woman taking many herbs and supplements, including bilberry, experiencing a serious bleed following surgery for breast cancer. The other herbs and supplements she was taking that may have contributed to the risk of bleeding include ginkgo, ginseng, and vitamin E.

The whole bilberry fruit, therefore, which contains both anthocyanidins and tannins (that may help stop bleeding) is likely safer if you are on a blood thinner than the isolated anthocyanidin extracts.