An infusion device capable of administering liquid medication at a continuous flow rate, and upon user demand delivers a controlled volume dosage of liquid medication at a higher dosage flow rate. The dosage reservoir remains empty until the user actuates it by selectively and temporarily removing the pressure source, such as a spring. During actuation, fluid rapidly flows from the medication reservoir to fill the dosage reservoir. After actuation, the pressure source exerts a higher pressure on the dosage reservoir than the medication reservoir pressure, which results in a temporary higher bolus flow rate. Thus, two distinct flow rates are achieved with one flow restrictor element.