This seems pretty fair and balanced:
Heartworm Treatment
Using Heartgard alone for 2 years is almost never the way to go. The dog will have adult worms in its heart for 2 year. Heartgard can sterilize worms and can help slow growth, but it does not necessarily stop growth and it does not stop the disease from progressing. Heartworm disease progressing for 2 years is not a happy thought and it's not what the AHS considers a good idea.
Whether or not SAMe should be given - couldn't say. I personally would do it if Immiticide was given as long as a vet told me it couldn't hurt. Millions and millions of dogs get some sort of drug daily or monthly though and don't receive SAMe or milk thistle and they are fine. So a healthy dog just taking a preventative once a month - I wouldn't spend my money on it.