Transdermal iontophoresis is an active drug delivery method that has the potential to transform treatment of conditions such as acute pain that require a succession of on-demand metered-dose drug deliveries. However, current monophasic iontophoresis methods fail to meet these requirements due to their inability to halt the passive diffusion of active agents when therapy is not required. We have developed a biphasic iontophoretic system to overcome these limitations. The viability of this system was assessed in an in vitro porcine skin preparation using FeCl₂ (127 Daltons), a charged molecule which can undergo both active and passive transdermal diffusion. The transport properties of the system were modeled using a Fourier Transform-derived optimum estimate transfer function. Using this model, experimental results showed good correlation to predicted values for both cumulative dose (R²=0.912, n=10), and density dose (R²=0.802, n=10). Results also showed the ability to effectively deliver the compound during active periods while minimizing delivery during inactive periods. While preliminary, our results suggest biphasic iontophoresis is a viable means of delivering on-demand drug therapy while minimizing unwanted off-demand delivery.