Yarrow is likely safe when taken by mouth in the amounts commonly found in food. However, yarrow products that contain thujone might not be safe.
Yarrow is possibly safe when taken by mouth in medicinal amounts. In some people, yarrow might cause drowsiness and increase urination when taken by mouth. When it comes in contact with the skin, yarrow might cause skin irritation.
Special Precautions & Warnings:
Pregnancy and breast-feeding: Yarrow is likely unsafe when taken by mouth during pregnancy as it can affect the menstrual cycle and might cause miscarriage.
There is not enough reliable information about the safety of taking yarrow if you are breast-feeding. Stay on the safe side and avoid use.
Bleeding disorder: Yarrow might slow blood clotting. In theory, taking yarrow might increase the risk of bleeding in people with bleeding disorders.
Allergy to ragweed and related plants: Yarrow may cause an allergic reaction in people who are sensitive to the Asteraceae / Compositae family. Members of this family include ragweed, chrysanthemums, marigolds, daisies and many others. If you have allergies, be sure to check with your healthcare provider before taking yarrow.
Surgery: Yarrow might slow blood clotting so there is a concern that it might increase bleeding during and after surgery. Stop taking yarrow at least 2 weeks prior to a scheduled surgery.