I use Sentinel for Dallas' heartworm meds & I give it monthly. He has only had 2 months so far but I have never noticed anything funny after giving it to him. I just place it in with his evening kibble & he's been fine every time.
Sentinel is actually two medications in one. It has the active ingredient for Interceptor and the active ingredient for Program. It is supposed to protect against heartworms and fleas. If you are still having problems with fleas, I would seriously consider changing to Interceptor only so you are not giving your dog an medication that is not working and then look for a more effective flea control.