Physicians' prescribing preferences were a potential instrument for patients' actual prescriptions of antidepressants

Neil M Davies, David Gunnell, Kyla H Thomas, Chris Metcalfe, Frank Windmeijer, Richard M Martin

Research output: Contribution to journalArticle (Academic Journal)peer-review

43 Citations (Scopus)
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To investigate whether physicians' prescribing preferences were valid instrumental variables for the antidepressant prescriptions they issued to their patients.

Study Design and Setting

We investigated whether physicians' previous prescriptions of (1) tricyclic antidepressants (TCAs) vs. selective serotonin reuptake inhibitors (SSRIs) and (2) paroxetine vs. other SSRIs were valid instruments. We investigated whether the instrumental variable assumptions are likely to hold and whether TCAs (vs. SSRIs) were associated with hospital admission for self-harm or death by suicide using both conventional and instrumental variable regressions. The setting for the study was general practices in the United Kingdom.


Prior prescriptions were strongly associated with actual prescriptions: physicians who previously prescribed TCAs were 14.9 percentage points (95% confidence interval [CI], 14.4, 15.4) more likely to prescribe TCAs, and those who previously prescribed paroxetine were 27.7 percentage points (95% CI, 26.7, 28.8) more likely to prescribe paroxetine, to their next patient. Physicians' previous prescriptions were less strongly associated with patients' baseline characteristics than actual prescriptions. We found no evidence that the estimated association of TCAs with self-harm/suicide using instrumental variable regression differed from conventional regression estimates (P-value = 0.45).


The main instrumental variable assumptions held, suggesting that physicians' prescribing preferences are valid instruments for evaluating the short-term effects of antidepressants.
Original languageEnglish
Pages (from-to)1386-1396
Number of pages11
JournalJournal of Clinical Epidemiology
Issue number12
Early online date24 Sept 2013
Publication statusPublished - Dec 2013

Structured keywords

  • Centre for Surgical Research
  • SASH


  • Instrumental variables
  • Clinical Practice Research Datalink (CPRD)
  • Physicians' prescribing preferences
  • Confounding by indication
  • Causality
  • Translational epidemiology


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