Not only do they break up clots, they inhibit the clotting cascade which can cause side effects such as uncontrollable bleeding and that is just a well known common side effect. Giving these "clot busters" uses an extensive checklist just to see if the patient is eligible. Most systems don't have paramedics administering these drugs, because once given there is no quick remedy antidote if you mess up and give it to a patient who shouldn't receive it.