Ever since my husband and I came out in rashes from our cats' toxic "tick treatment" given from the vet, we had to find alternatives.
I now use a pinch of sulphur (bright yellow powder you can buy from livestock places) in their food every day during tick season. Been doing this for 4 years and found no fleas and only about 1 tick per year on our cats, which is excellent considering we live in tick central country!
This is an old livestock remedy that apparently also works for humans!
I also sometimes add a bit of kunzea oil (diluted 1:4 with liquid coconut oil) on the nape of the cat's neck. Just for good measure... the sulphur is my consistent remedy though.