Biopharmaceutical classification system (BCS) is a scientific framework used to classify drugs intended for oral administration based on their solubility and permeability characteristics. It is a tool that is often used by regulatory agencies to determine the biopharmaceutical profile of a drug and its potential for bioavailability and drug absorption.

The BCS classifies drugs into four groups based on their solubility and permeability characteristics:

Class I — High solubility, high permeability: Drugs in this class have high solubility and high permeability. They are easily absorbed in the small intestine and have a high rate and extent of absorption. Examples include metformin, ibuprofen, and caffeine.

Class II — Low solubility, high permeability: The drugs in this category have low solubility but high permeability. These drugs are often limited by their low solubility and may result in poor bioavailability. Examples of class II drugs include ketoconazole, griseofulvin, and felodipine.

Class III — High solubility, low permeability: The drugs in this class have high solubility but low permeability. These drugs are often poorly absorbed and have low bioavailability. Examples of class III drugs include atenolol, cimetidine, and enalapril.

Class IV — Low solubility, low permeability: Drugs in this class have low solubility and low permeability. They are typically poorly absorbed and are often associated with reduced bioavailability. Examples of class IV drugs include diazepam, digoxin, and hydrocortisone.

The BCS is important because it can be used by regulatory agencies to simplify the drug development process, to identify potential drug development issues early on and to guide formulations to ensure optimal bioavailability. It can also help pharmaceutical companies to improve drug design and optimize drug efficacy.