@misc{9260,
  author = {Y. Vinogradova and C. Coupland and J. Hippisley-Cox},
  title = {Exposure to combined oral contraceptives and risk of venous thromboembolism: a protocol for nested case-control studies using the QResearch and the CPRD databases},
  year = {2014},
  journal = {BMJ Open},
  volume = {4},
  number = {4},
  pages = {e004499},
  issn = {2044-6055 (Print) 2044-6055},
  doi = {10.1136/bmjopen-2013-004499},
}