I give Safe Guard in liquid form to mine (the drug is the same as Panacur) - figuring for weights, when they need it. The foals get Safe Guard as their first worming at 30 days (single worming, not double dosed), then Strongid at 60 days before they get Ivermectin. Now our vet recommends alternating Strongid and Panacur for the first 6 months BEFORE using Ivermectin.
If I'm dealing with a new mini horse or Shetland pony that I don't know the worming history on or that arrives and suddenly doesn't do so well - I will do a Panacur Powerpac (or again - I do it with the liquid SafeGuard - easier for me!) unless it is a horse in the 1,000 lb area. I have also had yearlings suddenly look off and will do a quick 5 day "power pac" and they usually pick right up... Seems to go in cycles - every other year - haven't a clue as to why - and neither of my vets could figure it out either.