Fentanyl patch, any experience?

Cookie has a mass growing (most likely cancer) on his ribs, which seems to be causing lots of pain in his front leg.
The vet suggested (after every other pain killer we could think of) a fentanyl patch as the last attempt at making him comfortable, warning that if it doesn't work we will have to make a decision. The patch is supposed to be replaced every 72 hours but the vet warned it's human medication so might need a bit of trial and error to see how well it works for how long.
The first one was great and we thought it would be something that would give us much more time with him. He was playing with toys, eating better, not limping, pretty much back to his old self.
After the second one (replaced at 72 hours) he was limping all day, uncomfortable and that evening refused his food. That night he woke up about 4am and was sick over a dozen times. The next day he slept loads, ate with some encouragement and seemed a bit brighter. The following day he was pretty good again.
The third one he was off colour in the morning before the 72 hours to change it, so I changed it about 3 hours early. This time I was prepared for the nausia and had slippery elm but when he refused I resorted to zantac to help ease his stomach. He hates tablets but was that out of it that he just left the tablet in his mouth, the slippery elm was just running out of his mouth as he laid on the chair. I kept a careful watch over him and tried to spread the now paste like tablet across his tongue so it wouldn't be inhaled. It seemed to help and while he was uncomfortable, pacing, whining and unable to settle he wasn't sick. From about 6ish he was able to settle and slept most of the day again, eating when had fed small amounts and didn't seem too uncomfortable.
So today the patch if left 72 hours should have been changed about 3pm, I changed it at 8am and he's been more regurgitating his supper (AATU dry dog food) since about 9.30am than the usual hacking sick he normally does. He's miserable, limping, some whining, can't settle. I just want to hold him and tell him he's going to be okay but he seems to want space so I'm trying to leave him alone.

Has anyone used fentanyl and had similar? I'm terrified of loosing him, and not sure how I will cope, he is my everything. I'm trying desperately to hold it together as he gets really upset if I am and if I cry, but more importantly I know this isn't fair on him and while he doesn't seem to want to give up yet I don't think he ever will.

I'm going to try changing it in 48 hours, it shouldn't have worn off by then so we should be able to tell if it is simply changing the patch that is causing this or if it is withdrawal. The patch is supposed to steadily release the medication so shouldn't be any more in his system from a new patch until it runs out so I can't see why or how the new patch would effect him, and it would take a while for withdrawal to kick in... I think.

He's due back at the vets again tomorrow, so I can discuss it more there but I'd really appreciate some advice and any experiences anyone has.

And if you have read all that I thank you so very much.

SJ I am sorry I have no experience of the patches but just wanted to give you a hug because I think you need one (hug)x

Stop thinking about how much you will miss him and start thinking about how much discomfort and pain he is in, then consider your options.

Come on sweetheart time to be a brave soldier and let him go with some dignity.

We are not being hard and we know how you love him, we have all been through it but there comes a time when you have to look beyond your own loss, your little sweetheart deserves better.

I agree, that was my first thought, why won't she let him go with dignity and not let him suffer anymore. You are keeping him going for YOU not for him, would you like to suffer like that

The nausea seems to have run it's course, he's come and laid on the bed with me and seems more comfortable. My OH and I aren't ready to give up on him yet, it might seem harsh but if you saw him playing and scoffing his food yesterday you might be more supportive of working out a schedule with the patches.
I'm going to change the patch 48hrs from when it was put on, and if we have the same results we will have to think about what is best for him. My OH and I both love him more than anything, I'd do anything for him even if that means letting him go. I'm crying now and it's upsetting him, so I'm going to have to leave it at that.
I will do what we and the vet thinks is best, but I need to give him another chance with the patch. If this is just withdrawal then 48hrs should be okay, he can have a bit longer to play and be happy.

I'd like to thank Meg and Dobermonkey for being so kind with their words.

I am sorry if I sounded harsh but my love for animals and their suffering has to come first before our feelings. I always believe that an animal will tell you when it has had enough. If your little fella hasn't had that look in his eye yet then you will be right to fight for him but please look for that look in their eye that will tell you enough is enough. Please accept my hugs for your distress.

Oh, poor guy. He sounds so miserable. Is he an older dog? If so, I would let him go and I would not bother with the pain patches. If it's cancer, you're likely to have to let him go, anyway.

My old 15 year old champ got cancer and it was closing his nose and suffocating him. I watched him toss and turn at night, unable to breathe on his side and thus unable to get a restful sleep. He was put down later that week, he was suffering.

If you think the patch will truly help, give it another whirl, I'd hate to tell you to put him down before it's absolutely necessary. I have no experience with these patches, and this is temporary as given time, they will become ineffective as his pain worsens.

I will say this, with no cure or true pain relief in sight... you've gotta wonder if it's time to let go. But I know it's extremely hard for some, and despite the dogs welfare being more important than your feelings, you need to come to terms with it before you do it.

Take it from me, people who aren't prepared for the death of a loved one... even if they see it coming, can take an extremely hard fall afterwards. I don't want to see that happen to anybody else. Don't rush into anything, but sit with your boy in silence, look at him etc. for a long time, and THEN follow your heart.

Then you'll do what is best, whatever that may be.

We had our last dog put down she had had enough, like you said you could see it in her eyes. She was ready and when we took her to the vets they agreed and she went really peacefully.
I honestly don't think that he is there yet, he bounced to the door yesterday when my parents came over and even they mentioned how much of a fighter he is and he doesn't want to give in. He looked shocking before the patch and I was thinking this is it, but the vet suggested trying this as his eyes were still bright and he was just suffering with pain. He is moving much better, and it's just the day the patch runs out/is changed that is causing the big concern.
It wouldn't be so bad for me if he did have that look, at least I would know that he is ready to go, but right now I'm trying to fight for him as much as he is still fighting this himself.