@misc{9665,
  author = {L. Parkin and K. Sharples and R. Hernandez and S. Jick},
  title = {Risk of venous thromboembolism in users of oral contraceptives containing drospirenone or levonorgestrel: nested case-control study based on UK General Practice Research Database},
  year = {2011},
  journal = {Bmj},
  volume = {342},
  pages = {d2139},
  issn = {0959-8138},
  doi = {10.1136/bmj.d2139},
}