An implantable medication delivery device that is highly space efficient and can reliably and safely deliver controlled medication doses to a target site. The system includes a variable volume medication reservoir that is exposed to an ambient pressure equal to the ambient pressure at a system outlet port. A pump/valve subassembly is provided to draw medication from the reservoir and force a medication dose along a fluid transfer passageway to the outlet port. The pump/valve subassembly incorporates a safety mechanism, e.g., a balanced valve, which normally blocks medication flow to the outlet port and opens only in response to a pump induced unbalancing force. A protective shell is also disclosed for protecting the ambient reservoir.
