OK...."Power Pac" is only packaging the stuff inside is still 10% Fenbendazole. In order to do a horse any harm with Fenbendazole you would have to actually drown it in the stuff!!! HOWEVER....if you are worming a horses from an unknown source, killing all the worms stone dead in one go will cause it to colic- not seriously as the worms are degradable matter and will break down and pass out pretty soon, but I can assure you from first hand experience that the result is multi coloured and quite spectacular!!
So- buy ONE Power pac tube- unless you have a herd you will not need the five they market, or one ordinary Panacur tube. In either case you give 4.6mg per 100 lbs body weight, per day for five days. I usually top this off by giving the usual dose per bodyweight ( I use Kgs) of Ivermectin ten days after the last dose of Fenbendazole. Ivermectin will kill off anything (except tapes) that is left and get the bots too. Ten days is the life cycle of most worms so you get them as they hatch with this routine. This is done in October and April ( I ususally do the Spring one in Feb-March but April is recommended for the States) In between I worm every eight weeks with Ivermectin, and once a year with Praziquantel (Tape wormer). This routine should get everything that is known of at the moment!!
The tube is marked for ordinary worming and double dose worming- use the double dose for five days at the right weight for your horse.
In my dry high desert climate, worms aren't the problem they are in wetter ones-and I pick up the runs twice daily AND the manure in removed from the premises once weekly, courtesy of the trash pickup man! However, I do a double-dose-for-five-days-in-a-row deworming with SAFE-GUARD(which is exactly the same thing as Panacur)once a year. I use the big, multidose tubes(formulated for 4000 lbs. of horsey weight, I believe)for even better cost-effectiveness. All of my minis except the B mare weight tape at 275 lbs. or less, so I give a 500 lb. dose to the smaller ones, a 600 lb. dose to the 275+/- one, and an 800 lb. dose to the B(it really isn't hard to estimate those in-between dosages; just "eyeball" the space between the dosage markings and estimate what would constitute 1/4 or 1/2 or 3/4, etc., of the distance between the markings-remembering that this dewormer has a wide margin of safety in dosage....)With my APHA Paint mare, I give a 2000 lb. dose(remember, these are DOUBLE doses, by weight-given daily for the 5 days.) Actually, this regime is specificallyfor 'getting rid of ENCYSTED SMALL STRONGYLES, which is what "Quest", a NO-NO for minis, is touted as being "good at"-actually, I have read that the 5 day, double-dose of fenbendazole regime is MUCH more effective! The suggested deworming regimen for 'getting rid of' TAPEWORMS is a two day, double dose of STRONGID P(pyrantel pamoate) or its equivalent-OR, the use of the ivermectin WITH PRAZIQUANTEL formulations-as mentioned by rabbitsfizz-and, as she also stated, ivermectin is the only one that can 'get rid of' about every kind of internal(and some external!)parasite now known, except the ones mentioned above, in a single, proper dose.