Caverject and prostaglandin injections are similiar.  I had prostaglandin
injections to use 2 years ago.
The injection, depending on mix, produces an erection without stimulation.
Some times the erection will be very hard at the first 10 minutes and then
stay medium for about 2 to 3 hours.
Also sometimes a dull ache was felt for quite a while.  I am not sure if the
symptoms are differing for others.  If I used prostaglandin at night I would
be awake for some time with quite a large unit, and some pain.
In the am it was easier since after use, and getting up and showering and
walking seemed to lessen the pain and the erection would diminish.
Anyway, there may be some differences with either drugs but they do produce
erection without stimulation.
Doseage is what needs to be worked on.
I felt it was a great help regardless of some of the problems that "arose"
John Loomis

When it works, you get the start of an erection in about 10 minutes and a
full one in 20.  Mine softened after sex or after failure of sex.  However,
it can stay hard for hours.  That is why you work up to the right dosage.