Once a drug has been biotransformed, what is it called?

Once a drug has undergone biotransformation, it is referred to as a metabolite. Biotransformation is the process through which the body modifies the chemical structure of the drug, often to make it more water-soluble, which facilitates excretion. This transformation can result in either active metabolites, which may retain activity similar to the parent drug, or inactive metabolites, which are typically eliminated from the body.

The term "metabolite" encompasses all substances that are produced during the metabolic process, which can include various forms of the original compound as it is broken down and transformed by enzymes in the liver or other tissues.

In context, the other terms do not accurately represent the state of the drug post-biotransformation. An active ingredient refers to the original substance that produces the intended effect, while a prodrug is a compound that is inactive until metabolized into an active form. A surface agent, often related to detergents or emulsifiers, is unrelated to pharmacology and drug metabolism. Thus, "metabolite" is the most appropriate term to describe a drug after it has been biotransformed.