Not an IDU but I do have a scar on my head from a severe injury in February that needed to be addressed through plastic surgery. It was a month before I could be seen in public without scaring people.
I've found that the true explanation (that I was assaulted with an improvised weapon) has been the best in my professional dealings. The most common reaction I've gotten is "I hope they got the guy" and they did. At this point, you'd have to know me very well to see that a scar exists at all thanks to my brilliant surgeon.

But it was not that way at first. I was pretty scary looking until the two black eyes and nose faded. My surgeon knew that my insurance wouldn't cover anything cosmetic, so she utilized cosmetic techniques on me at the time. The last will be an elective procedure with lasers in September. I have to pay for that myself. It should be undetectable at that point.
(The scar is about 3/4 inches long and is vertical, near my left eyebrow into my forehead. The stitch marks are the last to heal but that's what has saved me from permanent disfigurement.)
I use this twice daily and have since the stitches were dissolved:
http://scarguard.com/
Basically it looks and smells like nail polish and is applied the same way, twice per day. It works on both old and new scars.
For a few small scars on my legs and hips, I use BioOil. You can get it at any drugstore and it seems like it would be good on arms. For sure, stay out of the sun - I can't go out in direct sun for more than 10 minutes without a wide-brimmed hat, which since I also have very fair pink skin, is a good idea anyways.
Mederma has done absolutely nothing. YMMV.