If you are taking pain killers as a result of a helmet, you are doing something seriously wrong. That said, when I was shopping for a helmet I spent hours trying on different ones and finally ended up with the first one I tried... odd. There are so many different shapes of helmets out there - but once you find the one that fits it is worth the effort BIG TIME.
If your helmet is causing you pain you haven't found the one that fits - keep looking. my 2c.
I take them only after long long rides.. i have a scorpion ex-400 and found that with eh stock screen after long rides, my head would be pounding.. but its really from the wind blast of the windscreen right into my helemt.. honestly i havent taken any yet this year for that problem.. i asume your head gets used to the exra preasure after a while...