I think since your puppy is 6 months old the pup will have to be
tested
for heartworms before you can start any heartworm preventative... you should have started it sooner.
Best of luck choosing a heartworm preventative.
We were using the heartgard preventative on all our dogs... our Rat Terrier ( who is 7 years old) was diagnosed with heartworms last week... and he's always been on the preventative. The heartgard company is paying for his treatment. I switched him over to Iverheart Max... it takes care of tapeworms also.