How are water-soluble drugs exit the body?

• A. They filter out through the kidneys into the urine
• B. They pass into bile and are excreted in feces
• C. They are metabolized to inactive forms in the liver
• D. They are exhaled through the lungs

Answer: (A)

Water-soluble drugs exit the body mainly through the kidneys into the urine. The kidneys filter the blood through the glomeruli, and small, polar molecules—especially those not tightly bound to proteins—pass into the filtrate. Tubular secretion can add more drug into the urine, facilitating clearance. Because these compounds are already polar, they are less likely to be reabsorbed and more likely to be excreted. In contrast, many lipophilic drugs rely on liver metabolism to become more water-soluble or are excreted via bile, and exhalation is useful mainly for volatile substances, not typical water-soluble drugs.