Think of a prescription drug like a chocolate cake. They all have chocolate in them (except those dodgy beet root chocolate cakes peddled by snake oil merchants lol) but they all have varying levels of the other ingredients. Generic medicines are the same - they are supposed to be "bio equivalent" so have the same effect but can use different binding agents (so in the cake oil rather than eggs etc) or things to bulk it out a bit.

Hope that DH is OK