In patients with renal impairment many drugs or their metabolites are excreted more slowly. It is therefore important to know what drugs will require dose reduction or are to be avoided altogether. An exception to this rule is gentamicin where careful use of a nomogram and monitoring of blood levels (peak one (1) hour after i.m. dose and trough before next dose) make its use relatively safe. (see nomogram on p. 21).

Most drugs are conveniently given at an interval equal to the half-life. If a drug is excreted exclusively by the kidney a reduction in Glomerular Filtration Rate (and hence creatinine clearance) will produce a corresponding reduction in drug excretion.

Thus if GFR falls by half the drug half-life can be expected to double; the dosage interval should therefore be doubled, while the loading dose and maintenance dose are not changed.

For practical purposes serum creatinine gives the most useful index of renal function. It does however fall with age and may not reflect the true state in the elderly, who may be assumed to have a GFR of 50ml/min even if serum creatinine is within normal limits.

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Drugs to be avoided or used with caution in renal failure
Active Ingredient GFR Dosage recommendations Comments
Potassium-sparing Diuretics 20-50 20-50 Moderate plasma K⁺ high risk of hyperkalaemia in renal impairment ; amiloride ex More ...
Pravastatin 10-20 - <10 Start at lower end of dosage range
Prazosin <10 Start with small dose Increased sensitivity to hypertensive effect and possible CNS toxicity
Primaxin(R) 20-50 Reduce dose
Primidone <10 Avoid large doses
Probenecid 10-20 Avoid Ineffective and toxicity increased
Procainamide 20-50 Avoid or reduce dose
Procarbazine 10-20 Reduce dose
Prochlorperazine see Antipsychotics
Promazine see Antipsychotics
Propranolol see Beta-blockers
Prophylthiouracil 20-50 Reduce dose
Pseudoephedrine <10 Avoid Increased CNS toxicity
Pyridostigmine 10-20 Reduce dose Excreted by kidney
Quinapril 20-50 Start with 2.5mg See also Captopril
Ramipril 20-50 Start with 1.25mg daily See also Captopril
Ranitidine <10 Use half normal dose Occasional risk of confusion
Salicylates see Aspirin
Salsalate see Aspirin
Salt Substitutes 10-20 Avoid routine use High risk of hyperkalaemia