Thrombolytic therapy breaks up dangerous clots inside your blood vessels, such as the one in the deep veins of your legs, called deep vein thrombosis or DVT. We inject a clot-dissolving medication into a blood vessel, which flows through your bloodstream to reach the clot. Alternatively, we may insert a small catheter into your blood vessels to directly target the clot. The tip of the catheter may carry the medication, or special attachments that mechanically break up the clot.