The entry of a drug into the systemic circulation following the administration of a drug product usually involves:1. the release of the drug from its dosage form into solution in the biological fluids at the absorption site, and2. the movement of the dissolved drug across biological membranes into the systemic circulation.The study of the various factors which can affect these processes and the application of this knowledge to obtain the expected therapeutic effect from a drug product when it is used by a patient are known as biopharmaceutics.