If someone is taking blood thinners like warfarin, what is the danger of consuming teas that thin the blood further?

It can lead to excessive bruising or bleeding due to potentiated thinning effects

When an individual is managing their blood viscosity with prescription anticoagulants such as warfarin, introducing herbal teas containing compounds that also possess blood-thinning characteristics creates a dangerous synergistic effect. Warfarin works by interfering with the body's clotting cascade. If a concurrently consumed herb further reduces the blood's ability to form clots, the overall anticoagulation effect becomes exaggerated far beyond the therapeutic target intended by the prescription. This state of excessive thinning significantly raises the risk of spontaneous or excessive bleeding, manifesting as easy bruising, prolonged bleeding from minor cuts, or potentially severe internal hemorrhage.