Absolutely, especially in the increasingly dark world of opioiphobic official job'sworths. If travelling abroad you're as well asking your GP or DSP to provide a letter stating any O.S.T. or opioid pain relief just to be on the safe side. It won't only ruin your own holiday if you're dragged in at customs then sent on the next plane home but also any friends or family who might be accompanying you.
If you have a letter with a contact number on it, you'll be able to relax before you even get there. Because of the rush & carry on trying to get tablets ( got them the night before & cleaned the pharmacy out of their stock as they didn't know to get them in for me ) I didn't have a letter. Imagine the look on my kid's faces (8 & 7 at that time ) if I'd been deported from the country where Santa lives!
As it was we were so far North that they didn't even have a proper airport just a hanger, a tractor with a trailer for hold luggage & one customs official stamping passports. Passengers just sorted out there own luggage by eye. It could have been a different story completely though, you never know unless you've travelled to a particular destination or not what might be waiting for you at the other end.
Remember the woman who was locked up in Tunisia, Egypt(?) for " allegedly " carrying in Tramadol for her boyfriend's pain issues? Didn't go well for her, I do remember thinking that it was a shit load of tramadol at the time & the story was rather sketchy. She said they'd been prescribed for her, if that was the case things may have been a lot different if she had a letter from the GP.
Always ask for a letter is the answer!