The present invention discloses modified-release oral dosage forms of an azabicyclo derivative or its pharmaceutically acceptable salts, solvates, esters, enantiomers, diastereomers, N-oxides and polymorphs; and processes for the preparation thereof. The modified release formulation comprises an azabicyclo derivative, at least one rate-controlling polymer and at least one pharmaceutically acceptable excipient which provides therapeutically effective plasma levels of the active ingredient for a period of up to 24 hours.