No problem flying with medical syringes in the carryon - that's where they belong especially given the higher probability of checked luggage getting lost this Summer.
In practice, we've rarely (if ever) been asked for any documentation in Europe but still, it is certainly better to have a paper from your MD should you meet a zealous security officer. No specific format that i am aware of, just stating that due to (disease), passenger X needs medicine Y to be injected by syringe.