Hi we are having to frontline all our dogs, but one of my girls she has just come into season and we are hoping to mate her will the frontline make her infertile. We just spotted a flea on them.

I wouldn't use the spot on (although I know some do.)

One hot summer I had a toy size pregnant bitch, and found a flea.   My vet said that the Frontline spray was safe.  I blasted the little beggar, and my bitch whelped with no problems.

would not use spot on if planning to mate,but spray is fine and I found it to be just as effective.