Is your prostaglandin part of a Caverject system? The Caverject is a self contained dry mix system. Caverject is much more expensive than just straight prostaglandin (also called alprostadil).
My experience was significant discomfort using the prostaglandin, I used dosages of .10, .15 and .20 ml. I finally went to the bi-mix that does not contain the prostaglandin. I had no discomfort, but the results were not as good.
I would be very interested to hear why your doctor considers the prostaglandin to be "safer" than the tri-mix. Ask the question and get a substantive answer please. He might be right, but I have never heard that before.
I expect you will get a faster, harder erection with straight prostaglandin. If you do not have pain problems than it will be the right answer for you.