Risk of falls associated with antihypertensive medication: self-controlled case series.

PURPOSE: To quantify the possible role of confounding in our earlier estimates of the risk of falls amongst older people who have been prescribed antihypertensives.
METHODS: Self-controlled case series analysis of 9862 individuals older than 60 years of age with a first fall recorded between years 2003 and 2006 and at least 12 months of recorded history before the first fall, taken from 386 UK general practices contributing data to The Health Improvement Network primary care database. Conditional Poisson regression to estimate incidence rate ratios for periods of exposure and non-exposure, adjusting for the effect of age.
RESULTS: Incidence rate ratio of first fall in days 1-21 of the first episode of thiazide prescription is 2.80 (95%CI 1.7-4.57). For beta-blockers, the rate ratio in day 22 onwards is 1.23 (95%CI 1.02-1.48) in the very first episode of medication and 1.21 (95%CI 1.02-1.42) in subsequent episodes but is not significant in other periods of exposure. The incidence rate of first falls in day 22 of any episode of calcium channel blocker prescription is 0.75 (95%CI 0.60-0.92).
CONCLUSIONS: The study provides evidence that at least some of our earlier estimates were subject to confounding. Nevertheless, thiazide prescription remains associated with an increased risk of first fall, and this effect is strongest in the first 3 weeks of prescription. Our study demonstrates that the case series method provides an effective way to assess the extent of residual confounding in case-control studies.