I did a Power Pack of Panacure for my Arabian, Djinn, at my vet's suggestion, simply because no matter what other worming method we tried, he always came up positive, sometimes heavily positive for ascarids. Even though he was six or seven, we just couldn't get rid of them. We even did a double dose of Anthelcide EQ followed by daily feed through Strongid and then, six months later, a double dose of Zimectrin. We kept him on feed through Strongid for about a year with periodic de-wormings, but he still came up quite positive for ascarids. Djinn was not around other horses, and the vet was rather stumped.
Anyway, we decided to Power Pack him, and it knocked him right off his pins. He had never before been sick, but he was sick after that; he was flat out down sick, and it took him months to regain his weight and sparkle. I actually never finished the process, since he got so sick so fast. I had about $400.00 of Panacure sitting around my house forever!