I found this:
I won't use Quest for several reasons. The FDA Center for Vet medicine keeps records for product related injuries. In 1997, they had 159 regarding Quest and it was #4 in the list for complaints that year. By 1998, there were 487 complaints making it #2 in the list. Complaints are received from both vets and consumers, documented and investigated. It is known that horses with heavy worm loads can be killed by it and because it requires such precise dosing, too little does nothing and too much can be lethal. It's also been linked to fetal deaths. If the horse has a heavy worm burden, it should be wormed with something like Panacur first and then Quest a couple of weeks later. The thing is that when Quest first came out, people weren't aware of this and probably still aren't and that's why horses are dying, not just mini's.
Much has been made about the posting on the Fort Dodge website regarding internet rumors about the product and that's their right to defend themselves. However, the FDA shows that there were a lot of complaints regarding the product and that there was also a large jump in numbers reporting. I should also point out that the active ingredient in Quest - moxidectin - was also used in ProHeart which was for heart worms in dogs. It was also made by Fort Dodge and it was pulled off the market by the FDA because by 2004 they had received 5552 reports of injuries to dogs and around 500 deaths. Same drug, same company - no thanks!
The problem with Quest is that even though it is effective at the recommended dose, it has a very small safety factor compared to other wormers. I've copied the following info from the link I listed:
Quest (moxidectin) is expected to cause physiological problems in a healthy horse at 5 times the effective dose. That is, if you give a healthy horse 5 times the dose recommended by the manufacturer, according to the horse's weight, you could expect to see problems (neurological
problems would be most likely, with Quest/moxidectin), or have the horse die.
By contrast, ivermectin (brand names Zimecterin, Eqvalan, Rotectin 1, Equimectrin, etc.) is not expected to cause problems until 60 times the effective dose is given.
Oxibendazole (brand/trade name: Anthelcide EQ) is not expected to cause problems until 60 times the effective dose is given.
Fenbendazole (brand names Panacur, Safe-Guard) may cause problems at over 100-200 times the effective dose, although apparently when testing for side-effects from fenbendazole, the researchers had difficulty producing
any problems at all, at any amount of overdose.
Pyrantel pamoate (brand names Strongid P, Strongid T, Rotectin 2) is safe for a healthy horse at up to 20 times the effective dose.
I have had readers make the comment that it would be difficult to mistakenly give a horse 5 times the recommended dose of Quest/moxidectin.
That is not the point. The point is that a worm medicine that is likely to cause obvious metabolic/physiological problems at 5 times overdose is likely much more toxic to the horse than a worm medicine that may cause obvious metabolic/physiological problems at 20, 60 or 100-200 times
overdose, and is much more likely to cause subtle metabolic/physiological problems that may not be immediately apparent to a human observer at the
recommended dose than another worm medicine that may cause obvious metabolic/physiological problems at 20, 60 or 100-200 times overdose. It is a matter of relative toxicity.
Also, when worming foals it would be fairly easy to give several times overdose of Quest/moxidectin, and there isn't much room for error. Source(s): http://www.horsecountrydirectory.com/wor…
all makes sense to me!