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Alright, quick recap — and I’ll ask you to do the thinking out loud. First, the directionality rule in one sentence: In iontophoresis, a drug is pushed away from an electrode with the same charge — positive drugs from the anode, negative drugs from the cathode. Now, why do added salts reduce drug delivery even when the current stays the same? Think of the electrical current like a fixed-size “ride-share” that must be fully booked. If your solution contains only your drug ions, most of the passengers are your drug, so more drug gets carried. But if you add salts, you add lots of extra ions — like sodium and chloride — that also want a seat. The current doesn’t get bigger. It’s the same number of seats. So your drug now shares the ride with these other ions. Result: the same current flows, but a smaller fraction of that current is carried by your drug. And that means less drug gets delivered. This is the idea of current-sharing, also called transference. Your drug’s “share” of the current matters just as much as the total current. If you remember only two things, remember this: One: Directionality. Same charges repel. Pick the electrode that pushes your drug into the skin. Two: Transference. Total current is split among all ions present. More competing ions — like from added salts — usually means less of your drug delivered at the same current. Now, your turn — answer these out loud. 1) If your drug is positively charged, which electrode should it go under, and why? 2) If delivery is low but you can’t increase current, what could you change in the formulation to help your drug get a bigger share of the current? 3) What’s one real-world source of “extra ions” in a system — like salts or buffers — that might quietly steal current from your drug?

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