I use FES and had exactly the same thing as you, I just became sensitised to the pads. There are some hypoallergenic pads you can get and they work fine.
Mine’s an Odstock device i get from Sheffield Hospital. I just called them, explained the problem and they now provide me with the new pads all of the time. No problems since changing!!
I have noticed however that they don’t last as long before you need to change them but that is the only downside!!
I use an FES device part of the time, but it makes my leg very itchy the longer I leave it on.
Once I’ve taken the device off, I use something called ‘Aveeno Anti-Itch Cream’ on the pad-sites. You can get this online at Amazon, and I find it far more effective than the E45 cream suggested by the Odstock clinicians.
And please do read the information sheet that whammel has mentioned.
I have been using the Blue Pals 50x50 electrodes ever since Odstock first set me up with my FES. Never had any reaction from them. And the Odstock service is great - when I am down to the last pack, a quick phone call will have some more through the letterbox within a very few days.
I will also put in a plug for Joe Green (author of that info sheet). I had my last appointment with him, and he spent 10 minutes tracing an intermittent fault - right down to a break within the moulded anti-strain bit on one plug. Very thorough, very efficient. I hope to see him again next month - but then they are all nice people at Odstock.