I agree with Steve that foreplay is crucial to getting decent erections after injections.
I use Viridal Duo (Alprostadil) for my injections, and intially found that 20mg was not very effective.
So, I decided to take a small cut from my normal Cialis tablet an hour or so before injection. This improved things a bit. So, I asked my clinic at UCLH here in London if it was OK to continue with this combination. No objection was raised, so now I take a complete 20mg Cialis as well as the 20mg injection. It has improved things significantly.
I have had no side effects, but the full erection still only lasts about 20mins. So, UCLH has suggested I go on to 40mg injection and continue with the Cialis at the same time, if this is needed.
As far as I can see (via Google), there have been quite a few thorough medical studies which show combination therapy to be highly effective when just one treatment isn't working.