Bilberry is generally well tolerated at typical supplement doses, and serious side effects are rare. Most people who take bilberry extract experience no problems at all. The risks that do exist center on its potential to affect blood clotting and blood sugar, its interactions with certain medications, and the lack of safety data for specific groups like pregnant or breastfeeding women.
Common Side Effects
At standard doses of 160 to 480 mg of powdered extract daily, bilberry causes few reported side effects. Some people experience mild digestive discomfort, including nausea, upset stomach, or diarrhea, particularly at higher doses. These symptoms are more common when bilberry is consumed as dried fruit (20 to 60 grams daily) rather than as a concentrated extract.
Commercial bilberry supplements are typically standardized to about 25% anthocyanidins (or 36% anthocyanins), but the actual content can vary widely between products. This inconsistency means your experience with one brand may differ from another, and higher-than-expected concentrations of active compounds could increase the chance of side effects.
Effects on Blood Clotting
The most significant concern with bilberry is its potential to affect how your blood clots. Bilberry may reduce platelet aggregation, which is the process that helps blood form clots to stop bleeding. On its own, this effect is mild. But if you take blood-thinning medications like warfarin, the combination can increase the risk of serious bleeding.
This interaction matters most in two situations: ongoing use alongside anticoagulant medications, and the period leading up to surgery. Experts recommend stopping bilberry and other supplements that affect bleeding at least two weeks before any scheduled surgical procedure. If you’re on blood thinners, letting your doctor know you take bilberry is important so they can monitor your clotting levels appropriately.
Blood Sugar Changes
Bilberry has a mild blood sugar-lowering effect, which researchers have observed in small clinical studies. In one trial of people with type 2 diabetes managed through diet and lifestyle alone, a single dose of bilberry extract (containing 36% anthocyanins) reduced both post-meal blood sugar and insulin levels compared to a placebo. A larger trial of 120 overweight participants taking anthocyanins for 12 weeks, however, found no significant difference in glucose levels between the supplement and placebo groups.
The practical takeaway: bilberry is unlikely to cause dangerous blood sugar drops in healthy people. But if you take insulin or other diabetes medications that actively lower blood sugar, adding bilberry could theoretically amplify the effect and push your levels too low. Monitoring your blood sugar more closely when starting bilberry supplements is a reasonable precaution.
Allergic Reactions
True allergic reactions to bilberry are extremely rare. Bilberry belongs to the Ericaceae family, the same plant family as blueberries and cranberries. Documented cases of allergy to blueberries are limited to a single published report, and researchers have not found clear cross-reactivity between Ericaceae berries and other common fruit allergens like those in strawberries. That said, any plant-based supplement can theoretically trigger an allergic response. Signs to watch for include hives, itching, swelling, or difficulty breathing after taking bilberry for the first time.
Pregnancy and Breastfeeding
There is essentially no clinical data on bilberry safety during pregnancy or breastfeeding. No studies have measured whether bilberry compounds pass into breast milk, and no research has evaluated effects on nursing infants. The NIH’s LactMed database, which tracks drug and supplement safety during lactation, states that no recommendations can be made about using bilberry products while breastfeeding. One historical note adds a layer of caution: bilberry was used in European folk medicine specifically to reduce milk production, though this claim has not been studied in modern trials.
Eating bilberries as a normal food in small amounts is different from taking concentrated supplements. The lack of safety data applies primarily to the higher doses found in extract form.
Long-Term Use and High Doses
Most clinical trials of bilberry have lasted 12 weeks or less, so the effects of long-term supplementation are not well characterized. Researchers have noted that the antioxidant and blood sugar-lowering activities seen in lab studies do not always translate to the same effects in the human body, partly because the concentration of active compounds varies between products and partly because of differences in how individuals absorb and metabolize anthocyanins.
Very high doses of bilberry leaf (as opposed to the fruit extract) have raised more concern in animal studies, with potential effects on liver function over extended periods. Most commercially available supplements use fruit-derived extracts rather than leaf preparations, but checking the label to confirm what part of the plant is used is worth doing if you plan to take bilberry regularly.