title: Gallbladder disease and use of transdermal versus oral hormone replacement therapy in postmenopausal women: prospective cohort study.
abstract: OBJECTIVE
  To determine whether transdermal compared with oral use of hormone replacement therapy reduces the risk of gallbladder disease in postmenopausal women.
  
  
  METHODS
  Prospective cohort study (Million Women Study).
  
  
  METHODS
  Women registered with the National Health Service (NHS) in England and Scotland.
  
  
  METHODS
  1,001,391 postmenopausal women (mean age 56) recruited between 1996 and 2001 from NHS breast screening centres and followed by record linkage to routinely collected NHS hospital admission data for gallbladder disease.
  
  
  METHODS
  Adjusted relative risk and standardised incidence rates of hospital admission for gallbladder disease or cholecystectomy according to use of hormone replacement therapy.
  
  
  RESULTS
  During follow-up 19 889 women were admitted for gallbladder disease; 17 190 (86%) had a cholecystectomy. Compared with never users of hormone replacement therapy, current users were more likely to be admitted for gallbladder disease (relative risk 1.64, 95% confidence interval 1.58 to 1.69) but risks were substantially lower with transdermal therapy than with oral therapy (relative risk 1.17, 1.10 to 1.24 v 1.74, 1.68 to 1.80; heterogeneity P<0.001). Among women using oral therapy, equine oestrogens were associated with a slightly greater risk of gallbladder disease than estradiol (relative risk 1.79, 1.72 to 1.87 v 1.62, 1.54 to 1.70; heterogeneity P<0.001) and higher doses of oestrogen increased the risk more than lower doses: for equine oestrogens >0.625 mg, 1.91 (1.78 to 2.04) v <or=0.625 mg, 1.76 (1.68 to 1.84); heterogeneity P=0.02; estradiol >1 mg, 1.68 (1.59 to 1.77) v <or=1 mg, 1.44 (1.31 to 1.59); heterogeneity P=0.003. The risk of gallbladder disease decreased with time since stopping therapy (trend P=0.004). Results were similar taking cholecystectomy as the outcome. Standardised hospital admission rates per 100 women over five years for cholecystectomy were 1.1 in never users, 1.3 with transdermal therapy, and 2.0 with oral therapy.
  
  
  CONCLUSIONS
  Gallbladder disease is common in postmenopausal women and use of hormone replacement therapy increases the risk. Use of transdermal therapy rather than oral therapy over a five year period could avoid one cholecystectomy in every 140 users.
authors.affiliation: Epidemiology Unit, University of Oxford, Oxford OX3 7LF. Bette.Liu@ceu.ox.ac.uk
authors.forename: Bette
authors.initials: B
authors.lastname: Liu
authors.forename: Valerie
authors.initials: V
authors.lastname: Beral
authors.forename: Angela
authors.initials: A
authors.lastname: Balkwill
authors.forename: Jane
authors.initials: J
authors.lastname: Green
authors.forename: Siân
authors.initials: S
authors.lastname: Sweetland
authors.forename: Gillian
authors.initials: G
authors.lastname: Reeves
mesh_terms: D000279:Administration, Cutaneous; D000284:Administration, Oral; D015331:Cohort Studies; D004739:England; D005260:Female; D005705:Gallbladder Diseases / Q000139:chemically induced* / Q000517:prevention & control; D020249:Hormone Replacement Therapy / Q000009:adverse effects*; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D008875:Middle Aged; D017698:Postmenopause; D011446:Prospective Studies; D012606:Scotland