If a drug has a volume of distribution (Vd) less than 1, it is classified as?

A drug is classified as hydrophilic when it has a volume of distribution (Vd) of less than 1. Volume of distribution is a pharmacokinetic parameter that indicates how extensively a drug disperses throughout body fluids and tissues. A low Vd, particularly one less than 1 L/kg, suggests that the drug remains primarily within the plasma compartment and does not extensively distribute into body tissues.

Hydrophilic drugs are typically water-soluble and tend to remain in extracellular fluid, which accounts for their limited distribution. This characteristic aligns with the behavior of hydrophilic compounds, which do not readily permeate membranes composed of lipid layers that separate body compartments. Thus, the classification of a drug with a Vd of less than 1 as hydrophilic is consistent with its low tissue distribution and solubility behavior.

In contrast, drugs that are classified as lipophilic typically have a higher Vd, indicating they readily distribute into fatty tissues and cellular membranes. The classifications of hydrophobic and polar do not specifically correlate to the significance of the volume of distribution in the same way, as hydrophobic refers to the affinity for lipids rather than water, and polar describes molecular characteristics rather than distribution in fluids or tissues.