I would say every 4 weeks. My cats do not go outside yet they still managed to get flees a few weeks ago when I let is slip on month, cue lots of vets visits and spraying of the house. Its better to be cautious than have a house full of fleas

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Owned By:

1 British Blue Kittens: Lola
1 working cocker spaniel: Bodie (Bo)

Winston the British Blue: 23/09/2008 - 26/12/2010 xx