My experience is the same as pattir7's. I have a horse on polyglycan and it is much, MUCH easier to give as an IV injection. I tried it once IM; as noted, it is quite thick and challenging to inject nicely into the muscle.
My vet prescribed Poly G IM every thirty days. Yes it is viscous, but if you use the proper gauge needle it is no more difficult to inject than anything else. My horse pays no attention what so ever during the injection and does not show any ill effect after wards. For what it's worth. Dose is 5 ML or half a small bottle.
I have used it both IM and IV. I had been using it IV, but my mare is extremely hard to get a needle in and will rear and strike so the vet recommended IM as they said it works just as well. I did it for two months and noticed that it did not seem to work as well as it had when injected IV for my mare. I went back to IV and she improved once again.