So when we first got my cat, Charlie, we were using frontline on him for a while. Then after a few months he got a sore patch on the back of his neck the day after we put it on him. Our vet then gave us Stronghold to try and that's been fine for over a year now, but we put it on him yesterday and suddenly he's got a sore patch again.
Is there any reason why this would just suddenly start happening again?

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^^ Charlie is cooler than you

How often do you use the treatments?

Monthly like the packet (and vet) tells us to. It cleared up in a couple of days when it happened the first time. I just thought it was odd since we've been using this one for over a year with no problem.

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^^ Charlie is cooler than you

Have you looked at the ingredients in each preparation? Sounds likely there's one or more common to both which is responsible for the reaction. You might have better luck if you chose an entirely different product next time.

I realise they stipulate to apply monthly but I like some others only use flea treatments as and when necessary ie when I notice an outbreak. It's just I wish to avoid chemicals whenever possible!

Is he an indoor or outdoor cat and do you have any other pets ? but yes agree with Ianthi that maybe try and stretch the lenght of time between treatments, my two are indoor cats and only pets and I haven't treated them for fleas in the 18 months I've had them.

He is an outdoor cat, which is why we do it monthly (though we're usually late in applying it, so it's just over monthly really). And we have no other pets, no.

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^^ Charlie is cooler than you

I haven't thought to look at the ingredients no. I'll do that in the morning. I never thought to though, since we've been using it for over a year with no reaction.

We don't quite apply monthly, we're usually a week or so late, but I realize that is still quite regular. However Charlie is an outdoor cat, so I guess he's much more likely to pick them up than an indoor cat would be.

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^^ Charlie is cooler than you