What does it mean when a drug is described as 'lipophilic'?

When a drug is described as 'lipophilic,' it indicates that the drug has an affinity for fats or lipids, meaning it can easily dissolve in lipid-rich environments and traverse cell membranes. Cell membranes are primarily composed of phospholipid bilayers, and lipophilic substances can diffuse through these layers more easily compared to hydrophilic (water-soluble) substances.

The implication of lipophilicity in pharmacology is significant. Lipophilic drugs tend to be absorbed well when taken orally and can reach various tissues in the body, including the central nervous system, due to their ability to cross the blood-brain barrier. Their fat affinity allows them to accumulate in fatty tissues or bind to different cellular components, influencing their distribution, metabolism, and excretion.

Other answer choices describe different properties of drugs but do not accurately convey what lipophilicity means. Being easily dissolved in water pertains to hydrophilic properties, binding tightly to proteins relates to the pharmacokinetics of drug distribution, and being effective only in the gastrointestinal tract does not address the characteristic of lipophilicity itself, which applies broadly across various systems in the body.