So I've had Dax on Sentinel for...pretty much ever since he's been old enough to take heartworm preventatives. And me being the smart person that I am went and bought a six month supply of the 11-25 pound dosage. Well (wake up Amanda) so far I've only used one, and Dax is, as of a couple weeks ago, almost 25 pounds, so...my question is I know it's SAFE to give him the lower dosage but will it be strong enough? I mean will those couple extra pounds just make it completely ineffective? I don't want my poor guy getting heartworms just so I don't waste my money, I'll buy more if I have to, I just wanted, if possible, to use the stuff I already have. I plan to call the vet in the morning to see, I just wanted to see if I could get some opinions from you guys about it for now.