Heartworms developing resistance to monthly preventatives

Ahh . . . I remember Filaribits! My dogs always liked them. Would that really be an acceptable/more effective alternative? I'm fuzzy, but I seem to remember there was more risk in giving them to an already infected dog than there is with the current preventatives? And wasn't testing recommended every six months for dogs on Filaribits? That seems like such a long time ago . . .