Pharmaco-morbidity linkage: a feasibility study comparing morbidity in two pharmacy based exposure cohorts.

STUDY OBJECTIVES The aims were (1) to compare discharge diagnoses and concurrent medication in a pharmacy based cohort of users of H2 receptor antagonists to those in a population of users of other drugs in the same period, who did not use H2 receptor antagonists; (2) to compare these results to those of a similar study performed with the Tayside record linkage scheme. DESIGN AND SETTING The study was a retrospective cohort study. The morbidity data from the only hospital in one medium sized city (62,000 inhabitants) were linked to the dispensing data of all five community pharmacies on an individual basis (April 1, 1986-December 31, 1989). In the absence of a unique patient identification number, data from pharmacies and hospital were linked by the combination of date of birth, gender, and general practitioner code. For every user of H2 receptor antagonists two controls were obtained from all patients who had not used these drugs, and matched for age (within 5 years), gender, and general practitioner. All discharge diagnoses which followed this first prescription up to December 31, 1989, in a patient in the index cohort, and during the same period in his or her matched controls, were included in the study. MAIN RESULTS In the index cohort (n = 2174) 341 persons were admitted (526 admissions) as against 398 persons (527 admissions) in the control cohort (n = 4348). There was increased morbidity in the index cohort, especially concerning the gastrointestinal system (peptic ulcers and malignancies, abdominal pain, gastrointestinal haemorrhage), but also concerning the musculoskeletal, respiratory, and circulatory systems. The morbidity in the last three groups corresponded with drugs used concomitantly by patients in the index cohort, so it was probably not causally related to the intake of H2 receptor antagonists but was rather an indicator of higher levels of morbidity in the index cohort. CONCLUSIONS The figures were grossly comparable to those of the Tayside record linkage scheme. Probabilistic linking with the patient characteristics of gender, date of birth, and general practitioner code can facilitate the undertaking of postmarketing surveillance studies.