Just thought I would toss in my personal experience here. We have two lage dogs that both take heat guard for heart worm prevention. It is $76 every 3 months ( we buy the 6 month box and split it between the two) if we buy it at the vets office. A few years ago I bought the heart guard online at petmeds.com and saved almost $20 on each box, but one of my dogs still ended up with heart worms. The treatment was $600

. If I had purchased the meds at the vets office then the heart guard company would have paid for his treatment, but because I purchased them online I had to pay OOP.
I now only buy the heart worm meds from the vets office. I know that it is the correct medication and I have a guarantee that if one of my dogs has heart worms I will not be on the hook for a huge bill.