The most common side effect of blood thinners is uncontrolled bleeding. That could be a cut that won't stop, pee that's red or brown, or bowel movements that are red or look like tar. Other signs are bruises you don't know how you got or a very bad headache. Call your doctor if you notice any of these.
Go to the hospital or call your doctor if you have a bad fall or a hard bump, even if you don't see any blood.