 Modified drug release dosage forms:  These dosage forms are designed to release drugs in a controlled manner, at a predetermined rate, duration and location to achieve and maintain optimum therapeutic levels of drug.

 sustained release (SR)  prolonged action  controlled release (CR)  extended release (ER)  modified release  timed release  long acting

Drugs that have neither very slow nor very fast rates of absorption and excretion Drugs that have uniform absorption in the GI tract Drugs that have small doses (less than 500mg) Drugs with a wide therapeutic index Drugs for chronic disease conditions

First-order kinetics Zero-order kinetics

The process that takes place at a constant rate independent of drug concentration involved in process Rate of process cannot be increased even the drug concentration is increased

The process that is directly proportional to the drug concentration involved in process Rate of process increases linearly with increase in drug concentration

Biopharmaceutical factors Dosage form selection

Stomach Small intestine Large intestine

Dissolution of drug in stomach may be affected by presence or absence of food

Basic pH, large surface area and presence of bacterial flora may affect the absorption of drug

Not much is known about drug absorption in this area although unabsorbed drug that reaches the area are metabolized by bacteria.

To minimize risk of rapid stomach absorption To release the drug immediately