I had a similar experience traveling with an injector. It does pay to have a letter written on the doc's professional letterhead though.
I have had to travel with a pre-loaded pegasys dose and had no problems. If you look at the TSA website, it tells you that you can travel with medications including all your injection supplies. I printed out the actual page from the TSA website (in case the screener did not know their rules) and had my doc write up a letter explaining that I was on treatment. I told the screener that I had an injector in my bag and he just said "no problem". I think they deal with this daily, especially considering the explosion of diabetes in our country. I have only traveled on domestic flights, but I'm sure the TSA website will discuss international flights as well. Good luck.