a method is disclosed for delivering a drug substantially - free of rapid precipitation from an osmotic device . the osmotic device comprises a semipermeable wall surrounding a compartment housing a drug that exhibits limited solubility under neutral and acid conditions , and a compound capable of evolving carbon dioxide in the presence of an acid in the environment of use . the method comprises , imbibing fluid through the wall into the compartment at a rate determined by the permeability of the wall and the osmotic pressure gradient across the wall , forming in the compartment a basic solution containing drug and compound , which solution is delivered from the compartment through the passageway , reacting the compound with the acid in the environment at the device - environment interface for evolving carbon dioxide , thereby , providing an effervescent suspension in the environment that delivers the drug in a finely dispersed form to the environment of use over time . also , a composition is disclosed comprising the drug and the compound .