I have been combing my six kittens daily to reduce the number of their fleas. I applied Stronghold at exactly 6 weeks of age. None of the kittens appeared to have any adverse reactions. I combed again the next day for fleas and every one I found was dead. Will continue to comb until the infestation appears to be gone, but this product seems to work well with no ill side effects. My order from Sierra pet meds arrived in a timely manner and every question I have had has been answered to my satisfaction almost immediately. I will definitely be a repeat cutomer.