I always put the insulin pens in with my liquids clear bag when going through security (if you're checking in a bag you can put some in there and it shouldn't be questioned). Needles and other "dry" items just stay in my bag. I also try not to take more than I need to minimise time out of the fridge - especially on long haul trips.
I was diagnosed 4.5 years ago and fly several times a year. Have not once been questioned about meds or any of my other kit, nor do I carry a doctors note. Clearly, you could be checked but I think it's pretty obvious what you're carrying and why!