For example, if it was intended that medical marijuana was for pain relief, then a person using it merely out recreation use, would be misusing the drug.
In this example it is very unlikely that marijuana used medically or recreationally would cause harm.
Now lets contrast this with medical morphine. A doctor may use the drug correctly or to within the best of his skilss, and intends to use it to provide medical pain relief. Thus the doctor is using the drug as it was intended, which is correct use. Although, considering the pharmacology of the drug, there is a great risk that the actual patient could be harmed.