Active Pharmaceutical Ingredient or API is the biologically active part of the drug present in different types of medications like tablets, capsules, injectables, creams, etc. In simple words, it refers to those ingredients of a drug that produce the intended results. As active pharmaceutical ingredients are very powerful, a small quantity does the job.

The name and quantity of an API in medication are mentioned on the over-the-counter drug's package. In some cases, the name of the medicine is the active pharmaceutical ingredient used in it. 

Active pharmaceutical ingredient intermediates are the raw materials or raw drugs responsible for therapeutic action. They are the active ingredients which are later converted into various forms such as tablets, capsules, suspensions etc. 

API acts as a drug itself. Advanced intermediates are advanced forms of drug intermediates.