Drugs and the Kidney: Nephrotoxins and Dosing in Renal Impairment

Why the kidney is so exposed

Everything that concentrates a drug also concentrates its danger. The kidney's whole job is to sample the blood and process what it finds, which is exactly what makes it vulnerable. It takes roughly 20–25% of the cardiac output, so a large share of any circulating drug passes through it. It filters at the glomerulus and then reabsorbs water along the tubule — which means a drug in the tubular fluid is progressively concentrated as the filtrate is squeezed down, sometimes to levels far higher than in plasma. The medulla runs on a knife-edge oxygen supply, so anything that trims renal blood flow tips it into ischaemia. And the tubular cells actively take up certain drugs to secrete them, pulling toxins inside themselves. Filtration, concentration, marginal oxygen, active uptake — the four features that make the kidney brilliant at clearance are the same four that make it a target. The Principles of Pharmacology section covers renal excretion and clearance as a concept; here we see its clinical shadow side.

Nephrotoxins by mechanism — the high-yield map

Don't memorise a flat list. Group the culprits by how they injure — the mechanism predicts the pattern. The first and most important group is haemodynamic AKI — the kidney is not poisoned, it is starved of pressure. The glomerulus filters because of the pressure difference across it, set by two tiny vessels: the afferent arteriole coming in and the efferent arteriole going out. NSAIDs block the vasodilatory prostaglandins that keep the afferent arteriole open, so it constricts and inflow falls. ACE inhibitors and ARBs dilate the efferent arteriole (by removing angiotensin II's grip on it), so pressure inside the glomerulus drops and GFR falls — a real hazard in bilateral renal artery stenosis or volume depletion, where the kidney was leaning on that efferent tone to survive. Each drug alone is usually tolerated. Put an NSAID plus an ACE inhibitor/ARB plus a diuretic together — the notorious triple whammy — and you knock out inflow, outflow pressure, and volume all at once. The Cardiovascular section teaches ACE inhibitors, ARBs and diuretics in depth; the point to carry here is what the combination does to the glomerulus, and the Inflammation section explains the NSAID–prostaglandin link at the afferent arteriole.

The second group is direct acute tubular injury — a true poison reaching the tubular cell. The aminoglycosides (gentamicin) are taken up by proximal tubular cells and damage them from within; amphotericin B injures tubular membranes; iodinated radiocontrast combines vasoconstriction with direct toxicity; cisplatin is classically tubulotoxic (which is why chemotherapy protocols hydrate hard); and tenofovir can cause a proximal tubulopathy. The third group is acute interstitial nephritis — not dose-related poisoning but an allergic, immune reaction to the drug, with the kidney's interstitium infiltrated by inflammatory cells; think penicillins, PPIs, and again NSAIDs, sometimes with rash, fever and eosinophils. The fourth group is crystal and obstructive nephropathy — the drug (or its metabolite) precipitates as crystals inside the tubules, especially if the patient is dehydrated: aciclovir given as a fast bolus without fluids, sulfonamides, high-dose methotrexate, and the old protease inhibitor indinavir are the textbook offenders. A fifth, miscellaneous bin covers osmotic injury and others. Learn the four main buckets and most nephrotoxins slot themselves into place.

The other direction: dosing when the kidney is failing

If a drug leaves the body through the kidney, a failing kidney lets it build up. Many drugs, or their active metabolites, are cleared renally. When the kidney can't clear them, each dose stacks a little higher on the last until the drug reaches toxic concentrations — a normal dose becomes an overdose over days. The principle of safe prescribing in renal impairment is simple to state: estimate the patient's renal function (eGFR or creatinine clearance), then adjust the dose down or lengthen the interval so the accumulation matches the impaired clearance. Some drugs are reduced in size; others are given less often; a few must be avoided altogether below a threshold. This is the clinical application of clearance and dose adjustment from the Principles of Pharmacology section — the abstract equation turned into a prescribing decision at the bedside.

A working list of drugs that demand care or avoidance in chronic kidney disease is worth carrying. Metformin doesn't harm the kidney itself, but it is renally cleared and risks lactic acidosis if it accumulates — so it is dose-reduced and then stopped below an eGFR threshold, and always withheld in acute illness or AKI (the Endocrine section covers metformin and eGFR thresholds). Digoxin has a narrow therapeutic index and is renally cleared, so impairment quickly tips it into toxicity — reduce the dose and check levels (the Cardiovascular section covers digoxin). Gabapentin and pregabalin accumulate and cause sedation. Opioids matter especially: morphine's active metabolite (morphine-6-glucuronide) accumulates in renal failure and causes deepening sedation and respiratory depression, so an alternative opioid is preferred. Low-molecular-weight heparin (LMWH) accumulates and over-anticoagulates; some DOACs need dose reduction or avoidance by eGFR. Antibiotics split two ways: nitrofurantoin becomes both ineffective (it can't concentrate in the urine) and toxic below a low eGFR, so it's avoided there; aminoglycosides and vancomycin must be dose-adjusted and monitored by drug levels (the Antimicrobials section covers aminoglycoside, vancomycin and nitrofurantoin dosing).

Sick-day rules and contrast prevention

The safest dose of a nephrotoxin during a dehydrating illness is often none. The single most practical tool for preventing drug-induced AKI is the sick-day rule: when a patient has an acute dehydrating illness — vomiting, diarrhoea, fevers, poor oral intake — temporarily stop the drugs that either injure the kidney haemodynamically or accumulate when it fails, and restart them once they are eating and drinking normally. The memorable shorthand covers metformin, ACE inhibitors and ARBs, diuretics, NSAIDs, and SGLT2 inhibitors. Holding them for a couple of days does far less harm than an episode of AKI. SGLT2 inhibitors carry an extra sick-day reason — the risk of euglycaemic diabetic ketoacidosis during acute illness — which the Endocrine section covers. Separately, for procedures using iodinated radiocontrast, prevention centres on identifying at-risk patients (existing CKD, diabetes, dehydration), ensuring good hydration around the scan, using the lowest necessary contrast volume, and holding other nephrotoxins across the procedure. The unifying message of the whole topic is a single clinical reflex: know the nephrotoxins, use renal function to dose everything else, and hold the risky drugs whenever the kidney is under threat.