Beefsteak

Hi Leah,
I don't know what your history is but I assume because you are posting here you have some sort of heart disease and the shoulder pain is related to it.
You could get a second opinion, or even try a chiropractor, as he may be able to provide pain relief without medication. Acupuncture is also possible, as it treats pain pretty well, again without more medication. (It might pay to first tell your GP if you seek these sorts of options.)
A friend of mine has heart disease and he feels it as pain in the upper neck region - quite a problem for him. As far as I know he has no specific medication to treat the pain.
I think it's possibly related to changed patterns of blood flow - but someone else may have a better answer. The heart may be affecting the nerves too. If you have a very low blood pressure I would discuss that point with your GP, as it might be too low.
Beefsteak.