True or False: Only ionized drugs typically cross cell membranes.

True or False: Only ionized drugs typically cross cell membranes.

• A. True
• B. False
• C. Not enough information
• D. Depends on the drug

Answer: (B)

Membrane permeability hinges on the drug’s ionization state and lipid solubility. Uncharged (unionized) molecules are generally more lipophilic and can diffuse through the lipid bilayer easily, while ions are charged and cross membranes poorly by passive diffusion. This means the ability of a drug to cross a cell membrane depends on how much of it is in the unionized form at the local pH, which is described by the drug’s pKa and the Henderson–Hasselbalch relationship. The statement is false because crossing the membrane is not driven by ionized species. In fact, ionized drugs typically have limited passive diffusion. Exceptions exist: very small ionized molecules can leak through or be assisted by transporters or channels, and some drugs cross via active transport mechanisms or by other pathways. Environmental pH also shifts the balance between ionized and unionized forms, affecting absorption—for example, a weak acid tends to be more unionized in the acidic stomach, aiding absorption there, whereas it may become more ionized in the more basic intestine, reducing absorption. So, ionized drugs crossing membranes is not the typical rule; unionized, lipid-soluble forms cross more readily, making the statement incorrect.

Membrane permeability hinges on the drug’s ionization state and lipid solubility. Uncharged (unionized) molecules are generally more lipophilic and can diffuse through the lipid bilayer easily, while ions are charged and cross membranes poorly by passive diffusion. This means the ability of a drug to cross a cell membrane depends on how much of it is in the unionized form at the local pH, which is described by the drug’s pKa and the Henderson–Hasselbalch relationship.

The statement is false because crossing the membrane is not driven by ionized species. In fact, ionized drugs typically have limited passive diffusion. Exceptions exist: very small ionized molecules can leak through or be assisted by transporters or channels, and some drugs cross via active transport mechanisms or by other pathways. Environmental pH also shifts the balance between ionized and unionized forms, affecting absorption—for example, a weak acid tends to be more unionized in the acidic stomach, aiding absorption there, whereas it may become more ionized in the more basic intestine, reducing absorption.

So, ionized drugs crossing membranes is not the typical rule; unionized, lipid-soluble forms cross more readily, making the statement incorrect.