I was wondering about the different types of monthly heartworm preventatives that are out there. Beni's vet perscribes Sentinel, but you need a perscription to refill it, and I was wanting to get some online because it was cheaper. The vet is very hesitant about online purchasing of dog meds, but are they really any different? I ordered revolution for my cat and it seemed OK.

Any recommendations? What do you all use?