There wont be a difference in strength shooting in your feet. But you will probably get a delayed rush as it take longer to get to your heart/lungs than if you shot elsewhere.
Shooting in your feet has a much higher rate of complications (infections, thrombosis) than using your arms so if you are having to resort to that it might be time to rethink your IV use. Take a look at a vein map and make sure you are well hydrated and warm to give yourself the best chance of finding something in your arms. Also read this - IV technique and complications - before you go any further..
Plugging or smoking (depending on your drug) might be a good plan if all your veins are shot.