Which statement best describes Sodium Chloride Equivalent (E) in isotonicity calculations?

• A. It is the amount of NaCl needed to achieve isotonicity for the drug.
• B. It is the amount of isotonic solvent required to dissolve the drug.
• C. It is the molecular weight of the drug.
• D. It is the pH adjustment required for isotonicity.

Answer: (A)

Isotonicity is about matching osmotic pressure to what the body fluids tolerate, so the Sodium Chloride Equivalent expresses how much NaCl would have the same osmotic effect as a given amount of drug. In other words, it translates the drug’s contribution to osmolality into an equivalent amount of NaCl, accounting for how many particles the solute forms in solution (its ionization). This lets you adjust the formulation to the isotonic level by adding or balancing with NaCl, without changing the drug amount itself. The other ideas—how much solvent is needed to dissolve the drug, the drug’s molecular weight, or a pH adjustment—don’t describe the osmotic comparison used to achieve isotonicity.