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Lower heart failure (HF) rates in individuals taking chlorthalidone vs amlodipine, lisinopril, or doxazosin were unanticipated in the Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial (ALLHAT). HF differences appeared early, leading to questions about the possible influence of pre-enrollment antihypertensive drugs. A post hoc study evaluated hospitalized HF events. During year 1479 individuals had HF, with pre-entry antihypertensive medication data obtained on 301 patients (63%). Case-only analysis examined interactive effects (interaction odds ratio [OR, ratio of ORs]) of previous medication and ALLHAT treatment on HF outcomes, eg, did treatment effect differ by pre-entry antihypertensive class? Among cases, 39%, 37%, 17%, and 47% were taking pre-entry diuretics, angiotensin-converting enzyme inhibitors, β-blockers, and calcium channel blockers, respectively. Interaction OR for year 1 HF for amlodipine vs chlorthalidone for patients taking vs not taking diuretics pre-entry was 1.08 (95% confidence interval [CI], 0.53–2.21; P=.83); for lisinopril vs chlorthalidone, 1.33 (95% CI, 0.65–2.74; P=.44); and for doxazosin vs chlorthalidone, 1.13 (95% CI, 0.57–2.25; P=.73). Controlling for other pre-entry antihypertensives yielded similar results. There was no significant evidence that pre-entry drug type explained observed hospitalized HF differences by ALLHAT treatment.