It is similar to osmosis in that they both involve the semipermeable nature of the cell membrane.
Because transmembrane equilibrium must be maintained, another unionized molecule must diffuse into the cell to repeat the process.
The non-charged molecules of the drug remain in equal concentration on either side of the cell membrane.
For example, Naproxen is a non-steroidal anti-inflammatory drug that is a weak acid (its pKa value is 5.0).
[citation needed] Ion trapping is the reason why basic (alkaline) drugs are secreted into the stomach (for example morphine), where pH is acidic, and acidic drugs are excreted in urine where the conditions are alkaline.
Examples of animal hormones subjected to ion trapping include Prostacyclin and Leukotrienes.