I have recently discovered I have a B12 deficiency and so I have been doing some research. I have heard people saying that the B12 injections caused them to have painful muscle spasms and this was seen as being an unwanted side effect of the injection. I have found that when you have the injections your blood cells start to repair rapidly. This process requires large amounts of potassium and when that potassium runs out you can get muscle pain and spasms. I just wanted to pass this on so that anyone experiencing this knows it is a sign you are getting better but need more potassium.