Osmotically driven active agent dispenser for use in an aqueous environment that pumps a solution of active agent, such as a drug, at a predetermined, constant rate. The dispenser is in the form of a coated tablet comprising: a core of an osmotically effective active agent composition; and an inexpandable wall that encloses the core, has a controlled permeability to water and includes means, such as brittleness or sites of structural weakness, that is responsive to the pressure generated within the dispenser by imbibition of water from the environment by the core through the wall to form at least one, and usually many, passageways in situ in the wall through which the active agent composition in solution is pumped osmotically at a rate that is substantially dependent of the rate at which water is being imbibed into the dispenser.