albumin

Albumin is the most abundant protein floating in your blood plasma — think of it as the bloodstream's main cargo carrier. Besides holding water inside the vessels and shuttling fatty acids and hormones, it also serves as the principal taxi for many drug molecules, especially acidic ones.

Made by the liver, albumin has a couple of well-studied drug-binding sites (often called site I and site II). Acidic and neutral drugs such as warfarin, phenytoin, sulfonamides, salicylates and many NSAIDs bind here. Because albumin can carry a large amount of drug, it strongly influences how much of a drug remains bound versus free, and therefore how the drug distributes and acts.

When albumin levels drop — in cirrhosis, nephrotic syndrome, severe burns, malnutrition or critical illness — the free fraction of highly bound drugs can rise. This usually triggers compensatory clearance, so steady-state free concentration may not change much, but transiently or for narrow-window drugs it can matter. Two drugs competing for the same albumin site can also displace each other.