Medications should decompose in about a year, in a typical compost pile. However, compost piles are not nuclear reactors, so they don't change one element into another element. So, if the medication contains mercury, or copper, for instance, then there will be mercury and/or copper in the finished compost.

eric, Do they contain mercury?

A bit of copper is vital.

People that practice organic growing would compost that manure before using it in the garden and should compost all manure before applying it to the garden because one may not know if wormers have been used. The amount of the wormer most likely would be fairly small and probably not enough to make that much difference in a good healthy soil.

In soils where synthetics are used it would male little difference since the synthetic materials would be a much greater source of harm then the manure.

The sum of all the breakdown and dilution steps makes it very unlikely that there will be an effect on soil. One thing that has not been mentioned is the huge mass of soil in the average garden. A 20 x 20 ft. plot contains about 10 tons of soil in the top 6". A dose of anything diluted into that becomes very small.

If you can locate the original packaging for the dewormer that was given to your horse, it might be possible to look up the active ingredient on line, and ultimately find the chemical formula. That formula would reveal if mercury or any other toxic elements are present.

Also, there's no guarantee that a compound that works on intestinal worms will harm earthworms at all. I do know that many if not all vet drugs that are expected to end up on the ground through manure and urine, undergo environmental safety testing, which includes earthworm toxicity studies. At least when I was in that biz a few years ago, it was already being done for pesticides and it was starting to be done on drugs like antibiotics given to livestock.

Thanks to everyone who has responded! Toxcrusadr has an interesting point about earthworm toxicity studies for vet drugs that will end up on the ground, so I did some Internet research based on that, and discovered articles like this one, which make me feel much better about composting my horses' poo: http://santacruzwire.com/index.php/maria-gaura/162-is-horse-manure-safe-for-organic-gardens.html

It's interesting to read the different ways that people use dewormers. At our barn, the vet comes quarterly for vaccinations, worming and Legend. Not everyone gets a vaccination each time, but everyone who uses that vet service is offered worming on that schedule.

Our vet service actually tries to discourage horse owners from automatically giving horses dewormer without knowing if the horses have worms. They place articles around the barn with the scientific and environmental reasons for this. In theory, I think they are absolutely right, but in practice, it's nearly impossible to for the stars to line up so that my horse poos at precisely the right time for me to scoop it up and rush it to the vet while it's fresh enough to test. Either the vet hospital is closed, I simply can't leave the barn, or the horse just didn't poo at the right time.