I used alprostadil for about 6 months.  I've had fairly good response, once I found the right dosage.  My doc started me at 20 units after giving me a test shot in the office and then examining my erection.  Soon found in the real world with my FWB it didn't make me hard enough for penetration.  So had to increase by 5 units each use, until I found the correct dosage. Which for me turned out to be about 90 to 95 units.  100 units is the max dosage.  You are only supposed to take only 1 shot in a 24 hour period and no more than 3 shots in a week.

After use I usually experience an uncomfortable ache and a hard-on that lasts an hour or more.  Due to the pain my Doc switched me to a different med called BIMIX, it does not contain any alprostadil.  He told me to start at 50 units.  I thought, Oh no, here we go again.  But I used the 50 units and glad I did.   Best sex of my life.   Lasted 2 hours, wow, may reduce the dosage a little next time.  No pain with this stuff either.  

Hopefully this is the cure for my ED.  I had the brachytherapy procedure for prostate cancer about 6 years ago and had erection issues ever since