We've been giving them for almost 2 yrs now. The only time that we had a reaction was right after we started them, Peanut vomited once.
These pills don't affect the eggs but as soon as the fleas hatch they need to feed -from what I understand-so they bite and die. In that way it breaks the life cycle
I have 5 furkids on these pills and only had the one reaction so I'm satisfied with the results,hope this helps