There is increasing interest in protecting the environment from avoidable risks. This includes awareness that medicine residues may enter the environment, with potentially unwanted effects.
The most common route for pharmaceuticals into the environment is through excretion- by both people and animals - after they have been taken. From there, they can find their way into soil or water.
In the case of a veterinary medicine, any application for marketing must include an Environmental Risk Assessment (ERA) that details any potential risk posed to the environment, for a veterinary medicine this can be a reason to refuse a marketing authorisation. With this in mind, the use of medicines in agriculture is strictly controlled to minimise the levels of pharmaceutical residues that reach the environment.