Equest contains the drug moxidectin and is one of the few wormers that can control the important encysted larval stages of small red worms. It has a long period of action against worms, helping to reduce contamination in the pastures for grazing animals. This means that it is less likely to lead to over-dosing and the need for repeated treatments. It is also a long-lasting drug with low toxicity and a wide margin of safety. Moxidectin is used in equine wormer products because it is more stable than other drugs in water solutions, and is particularly effective when used at the correct dose. This makes it an ideal choice for use in horse wormers. It can be used in both feed and water based products, and has been proven effective in both types.