Is the drop in potassium levels just an initial reaction to starting B12 or will there be continual need for potassium whilst continuing with B12?

There’s a continual need to have potassium around and use it as needed. But it depends on the person. Some people don’t need to need to take potassium at all. Others need to take it religiously every day. 

But you'll definitely want to have some potassium on hand at all times. I still have potassium, and I still take it. But it’s not as urgent as it once was and I can go without it. It does appear that it’s more necessary at the beginning as the body adjusts to the B12.