Which of the following substances is considered a prodrug?

Codeine is classified as a prodrug because it requires metabolic transformation to exert its pharmacological effects. Once administered, codeine is converted into morphine through the action of the enzyme CYP2D6 in the liver. This metabolic conversion is essential for codeine to provide its analgesic effects, as codeine itself has only a mild opioid activity compared to its active metabolite, morphine.

Prodrugs are typically inactive when administered and only become pharmacologically active after undergoing metabolism in the body. This characteristic is crucial for the therapeutic efficacy of codeine. In clinical settings, understanding the concept of prodrugs helps in predicting the drug's effect and dosage considerations, especially in patients with variations in enzyme activity.