What Are the Side Effects of Taking Bilberry Supplements?

November 21, 2025 •

4 min read

While bilberry is widely recognized for its potential benefits, a 2019 review noted that side effects are generally mild and uncommon when using the fruit extract appropriately. However, it is crucial to understand what are the side effects of taking bilberry supplements, especially concerning potential drug interactions and risks associated with improper use. This article clarifies the potential adverse effects and safety considerations.

Quick Summary

Bilberry supplements can cause mild side effects like nausea and stomach upset, but serious risks involve interactions with blood-thinning and diabetes medications, potentially causing bleeding or low blood sugar. Inappropriate use, particularly of the bilberry leaf, is toxic. Certain populations, including pregnant women, should avoid it.

Key Points

Drug Interactions: Bilberry can cause dangerous interactions with blood-thinning medications and diabetes drugs, increasing the risk of bleeding or severe hypoglycemia.
Leaf vs. Fruit: Bilberry fruit extract is relatively safe, but the leaf is toxic in high doses due to tannins and should be avoided.
Mild GI Effects: Less severe side effects can include nausea, abdominal discomfort, and temporary changes in bowel habits.
Not for Everyone: Pregnant and breastfeeding women, as well as children, should not take bilberry supplements due to insufficient safety data.
Pre-Surgery Precaution: To prevent interference with blood sugar control, discontinue bilberry use at least two weeks before any scheduled surgery.
Allergic Reactions: Though rare, allergic reactions like rash, itching, and swelling of the face or throat are possible and require immediate medical attention.

Common and Mild Side Effects

For most healthy adults, bilberry fruit extract is considered safe when taken in recommended doses for short periods. Any side effects experienced are typically mild and related to the digestive system. These adverse reactions are generally not severe enough to warrant stopping supplementation unless symptoms become persistent or bothersome.

Gastrointestinal Upset

Nausea: Some individuals may experience a feeling of queasiness or sickness after taking bilberry supplements.
Abdominal Discomfort: Mild stomach aches or cramping have been reported.
Dark Stools: The pigment in bilberries can cause stool to become darker, which is a harmless, non-serious effect.
Diarrhea or Gas: As with any supplement, some users may experience temporary changes in bowel habits.

Serious Side Effects and Significant Drug Interactions

Beyond mild digestive upset, more significant concerns arise when bilberry supplements are used by individuals with pre-existing conditions or those taking specific medications. The anthocyanosides in bilberry, while beneficial in some ways, are responsible for these more serious interactions.

Risk of Hypoglycemia

One of the most important considerations for individuals with diabetes is bilberry's potential to lower blood sugar levels. When taken alongside diabetes medications, this effect can be amplified, leading to hypoglycemia (dangerously low blood sugar). It is essential for diabetics to monitor their blood sugar closely and consult their doctor before using bilberry. Stop taking bilberry at least two weeks before any scheduled surgery.

Increased Bleeding Risk

The anthocyanosides in bilberry also possess anti-platelet activity, meaning they can slow blood clotting. This poses a significant risk for those taking anticoagulant or antiplatelet medications (blood thinners), such as warfarin, aspirin, or clopidogrel. Combining bilberry with these drugs can increase the risk of bruising and excessive bleeding. A healthcare provider should always be consulted before adding bilberry to a regimen that includes blood thinners.

Allergic Reactions

Although rare, some individuals may experience an allergic reaction to bilberry. Symptoms can include hives, rash, itching, swelling of the face, tongue, or throat, and difficulty breathing. If any of these occur, medical help should be sought immediately.

Bilberry Fruit vs. Leaf: A Crucial Distinction

It is vital to distinguish between supplements made from bilberry fruit and those made from the leaf. While fruit extracts are generally considered safe, the leaf contains high concentrations of tannins and is considered possibly unsafe, especially when taken in high doses or for extended periods. Long-term use of bilberry leaf can have toxic side effects, including severe weight loss, muscle spasms, and can be fatal. It is strongly advised to avoid bilberry leaf teas or supplements.

Comparison of Bilberry Supplement Risks


Risk Category	Cause/Mechanism	Populations at Risk	Action Recommended
Mild Gastrointestinal	General digestive sensitivity to new substances or high doses.	General population	Reduce dose or discontinue use if symptoms persist.
Hypoglycemia	Bilberry's blood-sugar-lowering effect.	Individuals with diabetes on medication.	Consult doctor, monitor blood sugar, stop before surgery.
Increased Bleeding	Anti-platelet activity of anthocyanosides.	Those on blood-thinning medication.	Consult doctor, increased monitoring for bruising.
Allergic Reaction	Allergic response to bilberry compounds.	Individuals with a sensitivity.	Seek emergency medical attention.
Toxicity from Leaf	High concentration of tannins in the leaf.	Anyone consuming bilberry leaf products.	Avoid bilberry leaf entirely.

Populations That Should Avoid Bilberry Supplements

Pregnant and Breastfeeding Women: The safety of bilberry supplements in pregnant and breastfeeding women has not been sufficiently studied. Therefore, it is recommended to avoid their use.
Children: Similar to pregnant women, there is insufficient data to confirm the safety of bilberry supplements in children. Avoidance is the safest approach.
Individuals with Diabetes: As mentioned, the risk of hypoglycemia is too significant to ignore.
Individuals on Blood Thinners: The potential for increased bleeding is a serious concern.
Before Surgery: Bilberry can affect blood sugar, which could complicate surgery. Stop use at least two weeks prior to any procedure.

The Importance of Professional Medical Advice

Because herbal supplements are not as strictly regulated as prescription drugs, their quality, potency, and purity can vary significantly. To minimize risks, it is essential to purchase products from reputable sources and ensure they are made from the fruit, not the leaf. Most importantly, always consult a healthcare provider before starting any new supplement regimen, especially if you have pre-existing health conditions or are on other medications.

Conclusion

While bilberry supplements are generally well-tolerated and offer potential health benefits, it is crucial to approach their use with caution. The risk of dangerous interactions with blood-thinning and diabetes medications, along with the potential toxicity of the bilberry leaf, cannot be overstated. By understanding the potential adverse effects, confirming the product's quality, and seeking professional medical advice, you can make an informed decision and prioritize your health and safety. For more information on bilberry, consult resources like the National Center for Complementary and Integrative Health (NCCIH).

Frequently Asked Questions

Bilberry fruit extract has been used safely in studies for up to 6 months at typical doses. However, long-term safety data is limited, and it's best to consult a healthcare provider for continuous use, especially if taking high doses.

Yes, mild gastrointestinal symptoms such as nausea, abdominal discomfort, and gas are among the most commonly reported side effects of bilberry supplements.

Signs of a serious allergic reaction, though rare, include rash, itching, swelling of the face, tongue, or throat, and trouble breathing. Seek immediate medical attention if these symptoms appear.

While bilberry may affect heart health and blood pressure, its primary interactions are with medications that affect blood sugar or blood clotting. There is no specific evidence of strong interaction with blood pressure drugs, but always consult a doctor.

Bilberry has properties that may lower blood sugar. For individuals with diabetes on medication, this can lead to dangerously low blood sugar (hypoglycemia) if not carefully monitored.

No, they are very different. While the fruit extract is generally safe, high doses of bilberry leaf are considered unsafe due to high tannin content and have been linked to toxic effects, including severe weight loss and muscle spasms.

Bilberry should not be taken with blood-thinning medications (like warfarin, aspirin) or diabetes medications (like insulin, glipizide) without a doctor's supervision due to potential bleeding and hypoglycemia risks.