If you have breeding bitches, the safety studies are iffy. You can see them here: http://www.sentinelpet.com/media/pdf/spectrum_product_insert.pdf Heartguard and Frontline Plus have good data. As does Interceptor Spectrum (though you can't get in the US). Lufenuron (flea control) accumulates in the fat. Safety studies are done, but my vet is hesistant because of that and potential long term effects of accumulation. There are so many options out there, for my guys I choose something else. If I had a spayed/neutered pet, I probably wouldn't think twice.
Repro vet recommended only heart guard (plus is fine) for HW and fipronil only products for fleas. I have Sentinel that covers HW , other worms, and tapeworms- fleas also. I will not be using that in pregnant and nursing bitches. Has not been tested for safety in pregnant and nursing dogs, I won't test it on mine.
