Getting Rid of Drugs: Renal Excretion & Elimination
A drug's journey ends where a river meets the sea: the kidney. Every minute, your kidneys sift the blood, deciding what to keep and what to send out in the urine. When they slow down — with age or disease — drugs that were perfectly safe yesterday can quietly build up to poison. This is the story of the body's last checkpoint, and why the same dose can heal a young patient and harm an old one.
An 82-year-old man is admitted with a chest infection and started on a standard dose of an antibiotic — the very same dose a fit 30-year-old would receive. Three days later he is confused, his balance is gone, and his hearing has dulled. No one gave him too much by the numbers. But his kidneys, quietly weakened by age, could not clear the drug fast enough, so each dose stacked on the last until the level turned toxic. The dose wasn't wrong for a drug — it was wrong for HIS kidneys. To prescribe safely, you have to understand how the body shows a drug the door.
The kidney's three moves
Renal excretion is the sum of three processes. The net amount of a drug that leaves in the urine is decided by three tubular events. Glomerular filtration: the glomerulus filters blood like a sieve, pushing small molecules and FREE (unbound) drug into the tubule — but protein-bound drug and blood cells are too big to pass. Tubular secretion: in the proximal tubule, active carrier pumps grab drugs from the blood and secrete them into the urine; there are separate pumps for acids and for bases, and these pumps can even clear protein-bound drug. Tubular reabsorption: as urine concentrates, lipophilic and unionized drug can passively diffuse back into the blood — undoing some excretion. Excretion = filtered + secreted − reabsorbed.
When penicillin was scarce and precious in the 1940s, doctors gave it with probenecid — a drug that blocks the acid-secreting pump in the tubule. By jamming penicillin's secretion, probenecid slowed its excretion, kept its blood level high for longer, and made every precious dose last. It's a perfect demonstration that tubular secretion is a real, competable process — and the same principle still prolongs some antibiotics today.
Urine pH and the ion-trapping rescue
Because only unionized drug is reabsorbed, changing the urine's pH changes how much a drug is trapped in the urine and flushed out — the same ion-trapping idea from absorption, now used as an antidote. Making the urine alkaline ionizes a weak-acid drug, trapping it in the tubule so it can't be reabsorbed, and speeds its removal. This is how we treat serious aspirin (salicylate) or phenobarbital overdose: alkalinize the urine with sodium bicarbonate to pull the poison out faster.
- Renal excretion = filtration + secretion − reabsorption.
- Only free (unbound) drug is filtered; secretion pumps can clear bound drug too.
- Secretion is competable — probenecid blocks penicillin's pump, prolonging it.
- Alkalinizing urine traps & removes weak-acid drugs (aspirin, phenobarbital overdose).
The other exit: bile and the recycling loop
The kidney isn't the only door. The liver can excrete drugs (often after conjugation) into the bile, which empties into the intestine. Sometimes gut bacteria then cleave off the conjugate, freeing the original drug to be reabsorbed back into the blood — a loop called enterohepatic recirculation. This recycling prolongs a drug's stay in the body and raises its half-life. It also explains a classic clinical worry: because some oral contraceptive recycles this way, a course of antibiotics that kills those gut bacteria could, in theory, reduce its reabsorption and its effect. Beyond kidney and bile, tiny amounts of drug also leave in the lungs (exhaled anaesthetics and alcohol — the basis of the breathalyzer), sweat, saliva, tears, and breast milk.
Why the elderly man got sick: clearance and dose
Now the opening case resolves. Drugs that leave the body mainly by the kidney depend on renal function — measured as the glomerular filtration rate or creatinine clearance. As kidney function falls with age or disease, the clearance of a renally-excreted drug falls with it, so each dose is removed more slowly and the drug accumulates. For drugs with a narrow safety margin that rely on the kidney — the aminoglycoside antibiotics, digoxin, vancomycin, and many others — we MUST reduce the dose or lengthen the interval in renal impairment, guided by the patient's estimated kidney function. The elderly man received a young man's dose through an old man's kidneys.
Before you prescribe, ask: 'How does this drug leave the body?' If the answer is 'mostly unchanged by the kidney,' then the patient's kidney function is part of the dose. Check an eGFR/creatinine clearance and adjust. The most common cause of drug toxicity in the elderly is a normal dose meeting a declining kidney.
- Thinking protein-bound drug is filtered. Only free drug is filtered — but it can be secreted.
- Giving a renally-cleared drug at full dose in kidney impairment. It accumulates to toxicity.
- Forgetting enterohepatic recirculation can prolong a drug long after the last dose.
- Assuming dialysis clears every drug. High-Vd or highly-bound drugs are poorly dialyzed.
An elderly patient with reduced kidney function is given a standard dose of a renally-excreted antibiotic. The most likely result is:
- Renal excretion = glomerular filtration + tubular secretion − tubular reabsorption.
- Only free drug is filtered; secretion is competable (probenecid + penicillin).
- Urine pH controls reabsorption — alkalinize to remove weak-acid poisons.
- Renally-cleared drugs need dose reduction as kidney function falls — the classic elderly toxicity.
- Katzung BG. Basic & Clinical Pharmacology — Renal drug excretion & clearance.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Renal & biliary excretion; enterohepatic recirculation.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Drug elimination & renal excretion.
- Guyton & Hall Textbook of Medical Physiology — Renal filtration, secretion & reabsorption.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Drug elimination & dosing in renal impairment.
