Drug misuse is when a medication is used inappropriately and not according to medical advice (wrong dosage, frequency, etc). Drug abuse refers to use of illegal drugs.
Explanation:
Drug misuse can be related to legal or illegal drugs. Although by definition, drug abuse refers to illegal drugs, one could argue that drug misuse could also be considered abuse.
Veterinarians also look at the human and environmental aspects. All of these things affect an animal like the certain influences from its environment and how humans can add to those influences