Blueberry may interfere with the effectiveness of insulin, oral drugs for diabetes, and herbal products that affect blood sugar levels. It may also block the absorption of drugs and nutrients.

Bilberry, the European cousin of Blueberry, may interfere with the effectiveness of insulin, oral drugs for diabetes, and herbal products that affect blood sugar levels. It may also block the absorption of drugs and nutrients. Bilberry may also increase the risk of bleeding when taken with drugs that increase the risk of bleeding. Some examples include aspirin, anticoagulants ("blood thinners") such as warfarin (Coumadin®) or heparin, anti-platelet drugs such as clopidogrel (Plavix®), and non-steroidal anti-inflammatory drugs such as ibuprofen (Motrin®, Advil®) or naproxen (Naprosyn®, Aleve®).