Probably a silly question, but I don't want to do anything wrong with my puppy. Almost six months old and I have been giving her Heartgard and Advantix for fleas. I now have some Sentinel, which is supposed to take care of both. Dilemma is that she is due for her heartworm prevent about two weeks earlier than her flea prevent. So when should I give the Sentinel? Will it be ok to give it to her earlier (when she's due for the heartworm prevent) even though it might be doubling up on the flea prevent? Or better to let her go a couple weeks late on the heartworm? Thanks for any opinions.