Under Fick's law, the absorption process is typically described as which order?

• A. Zero order
• B. Mixed order
• C. None
• D. First order

Answer: (D)

Diffusion-driven absorption follows a driving force: the concentration difference across the intestinal wall. Fick's law states that the flux is proportional to that gradient. As drug moves from the gut lumen into the blood, the amount left in the lumen decreases, so the driving gradient weakens over time. When the rate of transfer remains proportional to how much drug is still available to be absorbed, the overall absorption rate diminishes as time passes. That time-dependent decrease in rate is the hallmark of first-order kinetics. Zero-order would require a constant rate regardless of how much drug remains, which isn’t what simple diffusion predicts. Mixed-order would imply a more complex situation, such as changing transport mechanisms, not the standard Fickian diffusion scenario. So the absorption described by Fick’s law is best characterized as first order.