In iontophoresis, how is a therapeutic drug delivered across the skin, and what equation relates current, time, and dose?

• A. A small, constant direct current drives charged drug ions through the skin; Dose (mA-min) = current (mA) × time (min)
• B. Alternating current drives diffusion; Dose = voltage × time
• C. Pulsed current with high frequency drives release; Dose = current × skin impedance
• D. No current; passive diffusion; Dose = area × thickness

Answer: (A)

Iontophoresis delivers drugs across the skin by using a small, constant direct current to push charged drug ions through the skin barrier. The electric field causes electromigration of the ions from the electrode toward the opposite pole, so only ionized forms of the drug can be transported effectively. A steady direct current is preferred because it provides a consistent rate of transport; alternating current or pulsed high-frequency current would repeatedly reverse direction or create pulses that can reduce net movement, making delivery less predictable. Passive diffusion with no current would be slow and poorly controllable, especially for charged molecules.

The dose concept here is tied to the total charge that passes through the tissue, measured as milliampere-minutes. Dose (mA·min) equals the current (mA) times the time (min). This means doubling the current or doubling the time increases the delivered dose proportionally, while voltage or skin impedance alone does not define the dose in this framework. For example, 2 mA for 20 minutes yields 40 mA·min of dose.

In short, a small, constant direct current drives ionized drugs through the skin, and the dose is proportional to current multiplied by time.