Bilberry supplements are generally safe for most adults in food-like amounts, but several groups of people should avoid them or use extreme caution. These include people taking blood thinners or diabetes medications, anyone scheduled for surgery, pregnant or breastfeeding women, people with iron deficiency anemia, and those with allergies to related berries.
People Taking Blood Thinners
Bilberry contains compounds called anthocyanosides that have antiplatelet properties, meaning they can slow blood clotting. If you’re taking anticoagulant medications like warfarin, or antiplatelet drugs like aspirin or clopidogrel, adding bilberry to the mix could increase your bleeding risk. The interaction between bilberry and these medications is not fully predictable. Italy’s surveillance system for natural health products documented a case where bilberry concentrate juice altered warfarin activity in a way researchers described as “unexpected,” with the interaction mechanism still unknown.
The unpredictability is the key concern here. Because bilberry can affect clotting through a mechanism that isn’t well understood, your doctor has no reliable way to adjust your medication dose to compensate. If you’re on any medication that affects blood clotting, bilberry supplements are best avoided entirely.
People on Diabetes Medications
Bilberry can lower blood sugar, which sounds beneficial but becomes dangerous when combined with insulin or oral diabetes drugs. In a controlled study of men with type 2 diabetes, a single capsule of standardized bilberry extract (equivalent to roughly 50 grams of fresh bilberries) significantly reduced blood sugar and insulin levels after a meal. The effect likely comes from bilberry slowing carbohydrate digestion and absorption, while also improving insulin sensitivity.
If you’re already managing your blood sugar with medication, stacking bilberry on top could push your levels too low. Symptoms of hypoglycemia include dizziness, shakiness, confusion, and in severe cases, loss of consciousness. This doesn’t mean bilberry is off-limits for everyone with diabetes, but if you’re using medication to control blood sugar, you should talk with your prescriber before adding it. People managing diabetes through diet and lifestyle alone face less risk, though monitoring remains important.
Anyone Scheduled for Surgery
Because bilberry affects both blood clotting and blood sugar, it can cause problems during and after surgical procedures. Surgeons need predictable clotting to control bleeding, and anesthesiologists need stable blood sugar levels throughout the operation. You should stop taking bilberry at least two weeks before a scheduled surgery. This timeline gives your body enough time to clear the active compounds and return to baseline. If you’re taking bilberry and learn you need an urgent procedure, let your surgical team know.
Pregnant and Breastfeeding Women
There is essentially no safety data on bilberry supplementation during pregnancy or breastfeeding. The NIH states that little is known about whether bilberry is safe in amounts greater than those found in foods during pregnancy or while nursing. The National Library of Medicine’s LactMed database, which tracks drug safety during breastfeeding, found no published information on whether bilberry compounds pass into breast milk, how they might affect infants, or what they do to milk production.
One detail worth noting: bilberry was historically used in European folk medicine to stop milk flow. Whether this effect is real or simply anecdotal hasn’t been studied, but it’s another reason to be cautious if you’re breastfeeding and want to maintain your supply. Eating a handful of bilberries in a dessert is a different situation than taking concentrated supplement capsules. The concern is specifically about supplemental doses.
People With Iron Deficiency Anemia
Bilberry fruit contains tannins, compounds that bind to iron in your digestive tract and form insoluble complexes your body can’t absorb. This is the same reason tea and coffee are known to reduce iron uptake from meals. For most healthy adults, this effect is minor and manageable. But for people already struggling with iron deficiency anemia, regularly consuming tannin-rich supplements can make the problem worse.
The groups most vulnerable to this effect are women (particularly those with heavy periods), children under five, vegetarians, and anyone already diagnosed with low iron stores. An estimated 30 to 40 percent of women and young children develop iron deficiency anemia without supplementation, so adding a tannin-rich supplement into an already precarious iron balance is worth reconsidering. If you fall into one of these categories and want to take bilberry, spacing it several hours away from iron-rich meals or iron supplements can reduce the interference, though it won’t eliminate it completely.
People With Berry Allergies
Bilberry belongs to the Vaccinium genus, the same plant family as blueberries, cranberries, and huckleberries. If you have a known allergy to any of these fruits, you should avoid bilberry. Cross-reactivity within this family is well documented. A case report published in the Journal of Asthma and Allergy described a child who experienced anaphylaxis from blueberry (which is closely related to bilberry) and also tested positive for allergic reactions to cherries, kiwifruits, pears, litchi, cherry tomatoes, and pineapple. The common trigger was a protein called lipid transfer protein, which is found across many fruits.
This means the risk isn’t limited to people allergic to other Vaccinium berries. If you have a broader fruit allergy, particularly one involving stone fruits like cherries or peaches, bilberry could trigger a reaction. Allergic responses to bilberry can range from mild (itching, hives) to severe (throat swelling, anaphylaxis). If you’ve ever had an allergic reaction to any berry and want to try bilberry, starting with a very small amount under controlled conditions is the safest approach.
Bilberry Leaf vs. Bilberry Fruit Extract
Most bilberry supplements use fruit extract, which is standardized for anthocyanin content and is the form used in clinical studies. Bilberry leaf preparations are a separate product with a different safety profile. While earlier concerns suggested bilberry leaves might contain toxic compounds, more recent research has found no alkaloids or toxic diterpenes in bilberry leaves. Animal studies have also shown no signs of liver or kidney toxicity from bilberry polyphenols and anthocyanins at studied doses.
That said, bilberry leaf products have far less safety data behind them than fruit extracts. If you’re choosing a bilberry supplement, fruit-based extracts are the better-studied option. Regardless of which form you use, the cautions above still apply.