Abstract: In previous studies, compound metformin/glipizide elementary osmotic pump tablet was developed. Aim To discover the mechanism of drug release from it. Methods Three rate-limiting factors influencing drug release from dosage form (the semi-permeable membrane, tablet core and delivery orifice) were investigated. Results The influx of water that passed the osmotic pump tablet was almost equal to the metformin release rate, while it was greatly less than the drug dissolution rate from tablet core. The size of orifice from 0.4 mm to 0.8 mm had no influence on drug release. The osmotic pressure of tablet core was mainly caused by metformin. Conclusion From the developed model of osmotic pump systems, it can be seen that only the water influx through the membrane is a rate-limiting step, not tablet core dissolution rate and solution influx, and only when the core dissolution rate is equal to the solution influx, the zero order release is seen in the osmotic pump systems.

Key words: metformin, metformin, glipizide, glipizide, osmotic pump, osmotic pump, mechanism, mechanism