Which type of erectile dysfunction is the hardest to treat?

Unresponsive ED: No joke! These days, most cases of erectile dysfunction respond quite well to oral PDE-5 inhibitors such as Cialis, Levitra, (vardenafil) Staxyn, Stendra & Viagra. It comes down to choosing which you like best & can afford (they're not cheap). But not everyone responds nor can take these pills. Alprostadil works but more invasive, both as pellet in uretrha, and as shot into penis. That's probably most difficul