title: Systematic review with meta-analysis: the adverse effects of tobacco smoking on the natural history of Crohn's disease.
abstract: BACKGROUND
  Tobacco smoking is a well-established risk factor for the development of Crohn's disease, and this may lead to a more complicated disease course. However, recent evidence suggests that many patients with Crohn's disease are unaware of this fact.
  
  
  OBJECTIVE
  To perform a systematic review and meta-analysis of the effects of smoking on disease course in Crohn's disease.
  
  
  METHODS
  A search of MEDLINE, EMBASE and EMBASE classic was carried out (up to July 2015) to identify observational studies reporting data on smoking and rates of surgery or flares of disease activity in patients with Crohn's disease. Dichotomous data were pooled to obtain odds ratios (ORs) for flares of disease activity or need for surgery, with 95% confidence intervals (CIs).
  
  
  RESULTS
  The search identified 33 eligible studies. Compared with nonsmokers, smokers had increased odds of flare of disease activity (OR, 1.56; 95% CI, 1.21-2.01), flare after surgery (OR, 1.97; 95% CI, 1.36-2.85), need for first surgery (OR, 1.68; 95% CI, 1.33-2.12) and need for second surgery (OR, 2.17; 95% CI, 1.63-2.89). The odds of these outcomes among ex-smokers diminished upon smoking cessation, with ORs comparable to those among nonsmokers and, in the case of flare or second surgery, significantly lower than smokers.
  
  
  CONCLUSIONS
  Smokers with Crohn's disease have a more complicated disease course than nonsmokers, and quitting smoking may ameliorate this. Patients should be reminded of the detrimental effects of smoking on the course of their disease, and smoking cessation advice should be provided to reduce disease burden and costs in these patients.
authors.affiliation: Leeds Gastroenterology Institute, St. James's University Hospital, Leeds, UK.|Leeds Institute of Biomedical and Clinical Sciences, University of Leeds, Leeds, UK.
authors.forename: N
authors.initials: N
authors.lastname: To
authors.affiliation: Leeds Gastroenterology Institute, St. James's University Hospital, Leeds, UK.|Leeds Institute of Biomedical and Clinical Sciences, University of Leeds, Leeds, UK.
authors.forename: D J
authors.initials: DJ
authors.lastname: Gracie
authors.affiliation: Leeds Gastroenterology Institute, St. James's University Hospital, Leeds, UK.|Leeds Institute of Biomedical and Clinical Sciences, University of Leeds, Leeds, UK.
authors.forename: A C
authors.initials: AC
authors.lastname: Ford
mesh_terms: D003424:Crohn Disease / Q000503:physiopathology* / Q000601:surgery; D018450:Disease Progression; D006801:Humans; D012307:Risk Factors; D012907:Smoking / Q000009:adverse effects*