Trauma drug TXA good before 3 hours, not so good after 3 hours - why?

Tranexamic Acid (TXA)

Why is it that an antifibrinolytic drug that is given to reduce bleeding in severe trauma patients can sometimes cause the very thing it is intended to stop? Professor Rob Medcalf explains the paradox in a published commentary that throws light on how the fibrinolytic system (that removes blood clots) can be inadvertently turned on when it should be turned off.

Tranexamic acid, or TXA for short, was extensively tested in 2010, in a trial of severe trauma patients called CRASH-2. The results showed improved outcomes if TXA was administered in the three hours immediately after injury, but after three hours, more deaths were caused due to bleeding. This commentary summarises the results of a study from a group in the US describing how TXA is doing different things at different times following head injury.

Reference: http://www.bloodjournal.org/content/bloodjournal/125/16/2457.full.pdf