Hi there!

Noticed my kitten Blackjack has a bald patch on the back of his neck. I did apply his Advocate 'spot on' flee and worming treatment the other day. But he's never reacted to it before and the leaflet doesn't mention hair loss being a likely side effect. Or it affecting their skin in any way actually.
He does go outside so maybe he got into a scrap? But it doesn't look sore.

Anyone know of cats reacting to spot on treatments in this way?

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Yes localised hairless is a not uncommon side effect of spot on treatments. Assuming he's fine in every other way it should be nothing to worry about. I would mention it to your vet next time you are in and consider a different brand for his next treatment.