Cialis is Tadalafil, and Viagra is Sildenafil.
I just looked at the leaflet in a Tadalafil box, and it says feeling sick is rare, 1-10 of every 1000 patients.
You could ask to try Vardenafil or Avanafil, which are the other two PDE5 inhibitors. I don't know the cause of feeling sick, so I don't know if other PDE5 inhibitors are likely to have the same effect or not.
Given the length of time, you might also ask to try the vasodilator drugs which will generate erections even without working nerves. These are mostly injections (Caverject, Viridal, Invicorp) but there a couple of others (MUSE, Vitaros).