I have a friend who has to take blood thinners because her blood clots too easily and this can be dangerous. The thinners work well, but she has to go for blood test regularly. If the thinners work too well then the blood clotting disease turns into a situation where her blood gets too thin.
When this happens she is in danger of having internal bleeding if she gets an injury. The bad thing is that she might have internal bleeding and not realize what is happening until she is in serious trouble. That's why she has to have regular blood tests.