Pyrethroids are natural products. That does not mean they are safe! Toxic mushrooms and botulism toxin are natural; would you use those?

There seems to be a lower bar for the FDA to approve OTC meds than prescription ones like Advantage, even though Advantage seems to be safe and effective for most animals. I'm not sure about this, though.

I still think the best person to consult with about product safety is your vet, even though he/she may try to sell you a more expensive product.