An implantable drug delivery system includes a housing having a plurality of compartments each of which includes an opening on one side thereof and a floor generally opposite the opening. Also included are a plurality of flexible drug containment sacks, each having a mouth circumscribing an opening in the sack, with the sack being disposed in a respective compartment with the mouth affixed to the opening of that compartment to seal the inside of the sack from the inside of the compartment and to enable holding a drug formulation in the sack. A plurality of covers are disposed over a respective compartment opening for preventing the release of drug formulation contained in the sacks in the compartment covered, until ruptured or removed. A plurality of gas generating elements are disposed at the floor of a respective compartment under the sack in that compartment and are responsive to initiation signals for producing gas to force the sacks and drug formulations against the covers to thereby rupture or remove the covers and force the formulations out of the housing. An electrical circuit supplies initiation signals to the gas generating means to initiate production of the gas and thus discharge of the drug formulation.