What is the primary difference between transdermal patches and topical gels in terms of mechanism and drug properties?

• A. Patches provide localized delivery with immediate effect; gels provide systemic delivery with sustained flux.
• B. Patches provide controlled, systemic delivery via a sustained flux; require suitable lipophilicity and potency; gels deliver locally with less sustained control and are more dependent on site of application.
• C. Patches rely on high aqueous solubility; gels require no lipophilicity.
• D. Patches and gels operate identically in mechanism and drug properties.

Answer: (B)

Transdermal patches are designed to deliver drug through the skin into the systemic circulation in a controlled, sustained way. To achieve that steady flux, the drug must have suitable skin permeability, which typically means enough lipophilicity and sufficient potency so that a patch can reach and maintain therapeutic systemic levels over time. Topical gels, on the other hand, are meant to act locally at the application site; their drug release is less precisely controlled over time and systemic absorption is more variable and often secondary to local effect. So patches emphasize systemic delivery with a steady flux and appropriate permeability/ potency, while gels emphasize local action with less sustained control and more site dependence.