I would never use Biospot.
They had a mishap several years ago, and a ton of cats died because of it.
Poor cats. Fortunately, I have dogs.
None of my guys have had an adverse reaction and, like I said, it works great on the ticks, which is far more important to me then the fleas (which it works great on too).
Again, find what works for you. No matter what, we are putting an insecticide onto/into our dogs and some will react adversely to it. There's no helping that.
Some people also will use the drops every 3 weeks instead of 4.
DON'T DO THIS! Totally not good for your critter. Follow the directions and apply appropriately. Use a natural method to help with the last week if you think it is wearing off.
There are other all natural varients that can be used but I don't know how effective they are when used by themselves.