Does study design influence outcome?. The effects of placebo control and treatment duration in antidepressant trials

Psychother Psychosom. 2009;78(3):172-81. doi: 10.1159/000209348. Epub 2009 Mar 24.


Background: Clinicians and researchers synthesize data from randomized controlled trials (RCTs) of antidepressants to make conclusions about the efficacy of medications for depression. All treatments include nonspecific factors in addition to the specific effects of drugs, and study design may influence patient outcomes via nonspecific factors. This study investigated whether placebo control and treatment duration affect the outcome in antidepressant RCTs.

Methods: Medline and the Cochrane Database were searched to identify RCTs of antidepressants for major depression approved by the Food and Drug Administration. Included studies enrolled outpatient participants aged 18-65, lasted 6-12 weeks, compared an antidepressant to placebo or another antidepressant and were published in English after 1985. Excluded trials enrolled inpatients, pregnant women and subjects with psychosis or mania. Mixed-effects logistic regression models including study type (placebo-controlled or comparator) and study duration (6, 8 or 12 weeks) as fixed effects determined whether these factors affected response and remission rates.

Results: In the 90 trials analyzed, the odds of depression response (OR = 1.79, 95% CI = 1.45-2.17, p < 0.001) and remission (OR 1.53, 95% CI = 1.11-2.11, p < 0.001) were significantly higher in comparator relative to placebo-controlled trials. Trials lasting 8 (OR = 1.37, CI = 1.14-1.64, p = 0.001) and 12 (OR = 1.52, CI = 1.12-2.07, p = 0.008) weeks had significantly greater response rates than 6-week trials without differing themselves.

Conclusions: Response and remission rates to antidepressants are significantly affected by study type and duration. Clinicians and researchers must consider the study design when interpreting and designing RCTs of antidepressant medications.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Aged
  • Antidepressive Agents, Tricyclic / therapeutic use*
  • Depressive Disorder, Major / diagnosis
  • Depressive Disorder, Major / drug therapy*
  • Female
  • Humans
  • Male
  • Middle Aged
  • Monoamine Oxidase Inhibitors / therapeutic use*
  • Placebo Effect*
  • Psychiatric Status Rating Scales
  • Randomized Controlled Trials as Topic
  • Research Design
  • Serotonin Uptake Inhibitors / therapeutic use*
  • Time Factors
  • Young Adult


  • Antidepressive Agents, Tricyclic
  • Monoamine Oxidase Inhibitors
  • Serotonin Uptake Inhibitors