If the OP allegation that just three companies control the insulin market is correct, then it doesn't seem appropriate to use the term "efficient" to describe t

If the OP allegation that just three companies control the insulin market is correct, then it doesn't seem appropriate to use the term "efficient" to describe the equilibrium condition that exists without the monopsony. Of course, we'd all probably be better off with a well functioning market free from monopolies or monopsonies.