Some possible side effects from taking bilberry extract include upset stomach, dry eyes and dry mouth. Those who take this may also experience allergic reactions, states Drugs.com.
The medicine derived from the bilberry plant is extracted from its leaves and dried ripe fruit, notes the United States National Institutes of Health's MedlinePlus. Although the majority of people supplementing on bilberry extract experience only minor or no side effects, there are rare cases of severe allergic reactions to it.
People taking bilberry supplements should seek emergency treatment if classic symptoms of a severe allergic reaction manifest, suggests eMedicineHealth. These dangerous side effects include hives, inflammation of the tongue, lips or face, breathing discomfort and throat closing.