I think whether or not you get questioned depends very much on the country in question. The Americans are pretty paranoid since 9/11 and on two occasions I have seen passengers refused boarding because they had no proof that what they were carrying really was insulin (and both were US citizens, not visitors). In October last year I was challenged at Boston Logan, despite carrying my GP's letter, and told I would have to inject myself to prove to them the substance in the pen was what it was claimed to be (Byetta not insulin). I told them I couldn't do that because I had just taken my permitted jab and couldn't have another one for at least 6 hours. If I injected again the overdose would result in a severe hypo on the plane and I would probably leave the plane in a bodybag at the other end. Eventually I got them to call the airport doctor who explained the situation to them and I was finally allowed to continue.