Drug exposure risk windows and unexposed comparator groups for cohort studies in pharmacoepidemiology

Alex D. McMahon, Josie M. M. Evans, Mark M. McGilchrist, Denis G. McDevitt, Thomas M. MacDonald

Research output: Contribution to journalArticlepeer-review

32 Citations (Scopus)


Aim: To determine the appropriate size of risk windows in both exposed and unexposed subcohorts.

Method: Data was taken from a previous study of upper gastrointestinal haemorrhage and perforation. The length of each prescription for NSAIDs was estimated. The risk was calculated for the duration of a prescription plus increments of -30, -25, ..., +115, +120 (i.e. 31 increments). Ten unexposed groups were re-sampled for each increment (stratified for age and sex), using the same lengths of risk window as the exposed group. Mean risks and rate-ratios were calculated (per thousand person-years).

Results: The NSAID risk rose from 3.52 at -30 days to a peak of 5.82 at -15 days, and then decreased gradually to 2.83 at +120 days. Unexposed risk was variable for the negative increments, and decreased gradually from 2.16 at +0 days to 1.54 at +120 days. The rate-ratio rose from 1.55 at -30 days to a peak of 2.85 at -5 days, and then decreased to 1.85 at +120 days.

Conclusion: Risk windows should be the same as (or slightly less than) the calculated length of a prescription. Lengthy windows should not be used for unexposed comparator groups (the exposed windows may be randomly allocated).

Original languageEnglish
Pages (from-to)275-280
Number of pages6
JournalPharmacoepidemiology and Drug Safety
Issue number4
Publication statusPublished - Jul 1998


  • Cohort study
  • MEMO
  • Pharmacoepidemiology
  • Risk window
  • Unexposed group

ASJC Scopus subject areas

  • Epidemiology
  • Pharmacology (medical)


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