Pretty much all flea treatments marketed for pets are toxic in the same way that coffee and chocolate are to people: a high enough quantity will cause problems.
If health concerns are genuinely a problem, you may want to look at Diatomaceous Earth for flea control purposes.
What is Diatomaceous Earth?
Diatomaceous earth consists of fossilized remains of diatoms, a type of hard-shelled algae. It is used as a filtration aid, mild abrasive in products including toothpaste, mechanical insecticide, absorbent for liquids, matting agent for coatings, reinforcing filler in plastics and rubber, anti-block in plastic films, porous support for chemical catalysts, cat litter, activator in blood clotting studies, a stabilizing component of dynamite, and a thermal insulator.
How it works
Fleas and other insects with an exoskeleton (hard shell) are susceptible to the glass-sharp edges of the microscopic diatoms. The silica shards cut through the waxy exoskeleton surface, effectively drying out the flea, resulting in death to these types of insects and their larvae.
Because DE is not a poison or toxin, insects cannot build up immunity to it. Also, it does not reduce in potency: as long as DE is still present, it is just as effective as when you first applied it.
Provided you get food grade DE, it is completely safe to touch and eat (in fact, it is commonly used for pest control on food crops intended for people, so there is a chance that you ate some recently). However, it is not good to inhale.
Unless it is labeled 100% DE, it likely contains poisonous additives that your pet should not have. If the label says "do not use on food", then don't use it on your pet. You will likely need to seek out a natural food store or special order it online.
More information: http://www.fleacontrolbook.com/naturalfleacontrol/diatomaceous-earth-the-a-miracle-cure-for-flea-control/