Based on the mechanism it is described to work through, blocking the scar forming pathways, it seems like the medication has to be dosed after a fresh wound is made so that it is in the tissue while the tissue is healing and when the scar formation would be occurring. If the wound is done healing and scar formed already my guess is that it would not have the same effect.
for stretch marks I hear laser treatment is pretty solid for reducing their visibility and making them much less visible. I plan to try this sometime soon myself