How strong is a 15 milligram morphine tablet?

how well does morphine IR 15mg q 8 hr for my migrainres instead of having to go to the Hospital. I have taken one and nothing has helped, so then I took another--- wait another hour, nothing, I take another one? Should they have given me a larger dose? My Drs know I have a very very high pain tollerance,so I takes a lot of pain pills to get my neck pain