Physicians' prescribing preferences were a potential instrument for patients' actual prescriptions of antidepressants.
OBJECTIVE
To investigate whether physicians' prescribing preferences were valid instrumental variables for the antidepressant prescriptions they issued to their patients.


METHODS
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.


RESULTS
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).


CONCLUSIONS
The main instrumental variable assumptions held, suggesting that physicians' prescribing preferences are valid instruments for evaluating the short-term effects of antidepressants.
School of Social and Community Medicine, University of Bristol, 39 Whatley Road, Bristol BS8 2PS, UK; Medical Research Council Centre for Causal Analysis in Translational Epidemiology, School of Social and Community Medicine, University of Bristol, Bristol BS8 2BN, UK. Electronic address: neil.davies@bristol.ac.uk.
Neil M
NM
Davies
David
D
Gunnell
Kyla H
KH
Thomas
Chris
C
Metcalfe
Frank
F
Windmeijer
Richard M
RM
Martin
D000328:Adult; D000928:Antidepressive Agents / Q000627:therapeutic use*; D000929:Antidepressive Agents, Tricyclic / Q000627:therapeutic use; D011307:Drug Prescriptions / Q000706:statistics & numerical data*; D005260:Female; D058006:General Practice; D006801:Humans; D008297:Male; D017374:Paroxetine / Q000627:therapeutic use; D010818:Practice Patterns, Physicians' / Q000706:statistics & numerical data*; D015203:Reproducibility of Results; D017367:Selective Serotonin Reuptake Inhibitors / Q000627:therapeutic use; D006113:United Kingdom
D000928:Antidepressive Agents; D000929:Antidepressive Agents, Tricyclic; D017374:Paroxetine; D017367:Selective Serotonin Reuptake Inhibitors
Causality; Clinical Practice Research Datalink (CPRD); Confounding by indication; Instrumental variables; Physicians' prescribing preferences; Translational epidemiology