I have only taken mine with me on the camino once, and quickly discovered that the daily walking and new routine worked wonders for my pretty much chronic pain. So luckily I don't have to take it anymore, though I am considering buying a Paingone Plus TENS pen to take with me just in case. I learnt the hard/stupid way that having electrical cables sticking out under my clothing while going through security was a bad idea ... Leaving them in your pack might also be a bit silly, so I'd advise you leave it in hand luggage, easily available, so you can take it out and show them if they ask. I was lucky - one of the security men had a granny who used hers a lot and they let me through.