I've travelled internationally frequently, including testosterone, syringes, hGH, misc other peptides, etc. I always have them clearly marked with an RX to my name and include a physician's letter(s) in the case. In literally 100s of flights, nobody has ever looked at the meds, shrugged at the syringes, never opened the DR's letters; the only thing they're always super-excited about are the ice packs for the peptides, so at this point I preemptively take out the ice packs, hand them to TSA, explain that I'm traveling with temperature sensitive medication (which they're used to, because that includes everybody on insulin). TSA will check that they are frozen -- if they're NOT frozen they won't let you fly with them -- and aren't explosives, and then wave goodbye.
The rules and regulations regarding what molecules are legal in which jurisdictions are ridiculously tangled and complex. If you check how many people fly each day, and how many of those people have prescriptions with them, that percentage is quite high. The TSA's overall stance is: it's not really their job to sort all that out, it's incumbent upon you to know the laws of the jurisdiction(s) you are traveling to, and as long as you don't have something exciting like 30-50 vials of something, 1000s of pills, half a kilo of mysterious powder, they really don't care. They only care about the ice packs which somehow may constitute a danger to other passengers or the crew, if they're not frozen (shrug). They will wipe those chemicals on the ice packs and test them, and then wave you through.
If you have global entry, they don't even do any of that, they just wave you through.