My mom just had to put down her 3yr.old black lab because of heartworms.He had them so bad the vet wouldn't even consider treatment because he was so bad.Putting him down was her only option.Needless to say she's heartbroken and the guilt she feels for not having him on a preventative is overwhelming.My Kujo gets it faithfully every month.No matter what brand you and your vet decide on it should definitley be given especially if you live in an area where mosquitos are a problem.It's horrible the way dogs suffer from it.