Ditto, ditto, and ditto taking a fecal sample to the vet and letting the vet tell you what and how much to worm with.
There are legitimate reasons to overdose horses with wormers but it should NOT be done "just because" someone on a forum said so or "just because" the previous owner decided it was the right thing to do.
I double-dose with pure
Ivermectin in mid to late summer if I see I am dealing with neck threadworms.
Neck threadworms are microfiliae from the bites of the same Midge Fly that causes sweet itch; they are not related to neglect in a worming schedule. They do not show up in a fecal count and only sometimes show up in a skin culture. They are sometimes the cause of moon blindness.
I do not double-dose every horse I own, just the ones that are showing me the specific skin symptoms neck threadworms produce. It's something a person learns over time OR ask the vet.
Then there's the Panacur PowerPacks, which I am against but who am I to say. Again -- a subject for discussion with the vet.
Like anything else, under the correct circumstances
, double-dosing can serve a good purpose ---- the OP's conditions do not sound like a good purpose and ^5 to the OP for asking the question
The horse might be dealing with ulcers - has it been checked for them? Ulcers will cause a horse to not keep weight on - another reason to not be arbitrarily shoving wormer down a horse as the wormer only serves to aggravate the ulcers and the vicious "skinny cycle" goes on.