Cranberry extract is generally safe, but some people may experience stomach upset, diarrhea, or an allergic reaction. Consult a doctor if you experience these symptoms. Who should avoid or use cranberry extract cautiously? 1. Individuals who are allergic to cranberries, blueberries or other related fruits should avoid cranberry extract to prevent allergic reactions. 2. Cranberries contain a significant amount of oxalate, a substance that can contribute to the formation of kidney stones. People with a history of kidney stones or who are at risk for developing them should use cranberry extract with caution. 3. Cranberry extract can enhance the effect of blood thinners, which can lead to an increased risk of bleeding. People taking anticoagulants should consult their doctor before using cranberry extract. 4. Cranberry extract and cranberry juice may contain sugars. People with diabetes should be aware of their sugar intake and choose products without added sugar.