Objective: To compare advice on dosage adjustment for renal impairment provided by four commonly used secondary pharmacotherapeutic sources. Design: Systematic comparison of the definitions of renal impairment, recommendations for dosage adjustment, and the evidence in support of these recommendations in four information sources. Data sources: British National Formulary, Martindale: the Complete Drug Reference, American Hospital Formulary System Drug Information, and Drug Prescribing in Renal Failure. Review methods: Two reviewers independently extracted data on recommendations for dosage adjustment for impaired renal function of 100 drugs often used in our hospital. Results: The four sources differed in their recommendations for adjustments of dosage and dosing interval. They vary in their definitions of renal impairment; some are qualitative and remain unclear. All sources provide only a general description; the methods on which the advice is based and references for original data are rarely presented. Conclusions The remarkable variation in definitions and recommendations, along with scarce details of the methods used to reach this advice, makes the available sources of drug information ill suited for clinical use. The methods used to retrieve information and use data should be described and made available to the reader. Advice on drug prescription, dose and dosing interval, contraindications, and adverse effects should be evidence based.