People with allergies to plants in the Ericaceae family or to anthocyanosides may have reactions to bilberry. However, there is a lack of reliable published cases of serious allergic reactions to bilberry.

Bilberry is generally believed to be safe in recommended doses for short periods of time, based on its history as a foodstuff. There are no known reports of serious toxicity or side effects, although if taken in large doses, there is an increased risk of bleeding, upset stomach, or hydroquinone poisoning.

Based on human use, bilberry fresh fruit may cause diarrhea or have a laxative effect. Based on animal studies, bilberry may cause low blood sugar levels. Caution is therefore advised in patients with diabetes or hypoglycemia, and in those taking drugs, herbs, or supplements that affect blood sugar. Serum glucose levels may need to be monitored by a healthcare provider, and medication adjustments may be necessary.

In theory, bilberry may decrease blood pressure, based on laboratory studies.

With the use of bilberry leaf extract, there is a theoretical increased bleeding risk, although there are no reliable published human reports of bleeding. Caution is advised in patients with bleeding disorders, taking drugs that may increase the risk of bleeding, or prior to some surgeries and dental procedures.

There is not enough scientific evidence to recommend the safe use of bilberry in pregnancy or breastfeeding, although eating bilberry fruit is believed to be safe based on its history of use as a foodstuff. One study used bilberry extract to treat pregnancy-induced leg swelling (edema), and no adverse effects were reported.

Fresh berries 55 to 115 grams three times daily, or 80 to 480 milligrams of aqueous extract three times daily by mouth (standardized to 25% anthocyanosides) have been used traditionally.

Dried fruit 4 to 8 grams by mouth with water two times per day has been used traditionally, or decoction of dried fruit by mouth three times per day (made by boiling 5 to 10 grams of crushed dried fruit in 150 milliliters of water for 10 minutes and straining while hot), or cold macerate of dried fruit by mouth three times per day (made by soaking dried crushed fruit in 150 milliliters of water for several hours). Experts have warned that patients should use dried bilberry preparations, because the fresh fruit may actually worsen diarrhea.

Some experts recommend using a mouthwash gargle of 10% dried fruit decoction as needed for mucus membrane inflammation.

There is not enough scientific evidence to recommend the use of bilberry in children.

Based on animal research, bilberry may lower blood sugar levels. There is a lack of reliable human studies in this area. Caution is advised when using medications that may also lower blood sugar. Patients taking drugs for diabetes by mouth or insulin should be monitored closely by a qualified healthcare provider. Medication adjustments may be necessary.

Based on human use, bilberry may increase diarrhea when taken with drugs that cause or worsen diarrhea, such as laxatives or some antibiotics. Bilberry theoretically may 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®). There are no reliable published human reports of bleeding with the use of bilberry. Based on theory, bilberry may further lower blood pressure when taken with drugs that decrease blood pressure.

Based on early laboratory study, berry extracts have been shown to inhibit H. pylori, an ulcer producing bacteria, and enhance the effects of the prescription drug clarithromycin (Biaxin ®).

Bilberry may also interact with anticancer agents, liver-damaging agents, and estrogen-containing medications. Consult with a qualified healthcare professional, including a pharmacist, to check for interactions.

Based on animal research, bilberry may lower blood sugar levels. Although there is a lack of reliable human study in this area, caution is advised when using herbs or supplements that may also lower blood sugar. Blood glucose levels may require monitoring, and doses may need adjustment.

Based on theory, bilberry may lower blood pressure further when taken with herbs or supplements that decrease blood pressure.

Based on theory, bilberry may increase the risk of bleeding when taken with herbs and supplements that are believed to increase the risk of bleeding. Multiple cases of bleeding have been reported with the use of Ginkgo biloba, fewer cases with garlic, and two cases with saw palmetto. Numerous other agents may theoretically increase the risk of bleeding, although this has not been proven in most cases.

Based on traditional use, bilberry may increase diarrhea or laxative effects when taken with herbs and supplements that are also believed to have laxative effects.

Consuming bilberry with quercetin supplements may result in additive effects. Cooking bilberries with water and sugar to make soup may decrease the amount of quercetin by 40%. Berries contain resveratrol, which has been studied as an antioxidant, for cardiovascular disease, and for cancer and may have additive effects when taken with supplements like grapeseed.

Bilberry may also interact with anticancer agents, antioxidants, liver-damaging agents, and herbs or supplement with hormonal properties. Consult with a qualified healthcare professional, including a pharmacist, to check for interactions.