We have had two precious BSH cats die a few hours after being given the recommended dose of Comfortis (an oral flea control product containing an insecticide called Spinosad). A third cat also died suddenly 2 weeks after dosing.
The manufacturers (Elanco) are investigating the incident and claim the product is safe for use as directed. The autopsy confirms that our cats (age range between 2-3 years) were healthy and died very suddenly.
If you have concerns after reading this please discuss your methods of flea control with your vet.