Seems a bit of an overreaction from the GP. I wonder why he didn't just say to reduce your insulin by a few units each day or every other day until you don't ever go below 4; that's a much more scientific and logical approach (sorry but I'm an engineer by training!). 50 units does seem a bit high so perhaps you are a bit overweight and could usefully diet down a bit by lowering your carbs; just guessing? If you do lose weight then the insulin needs would drop anyway due to reduction in insulin resistance