To cite flexsurv in publications use:

  Christopher Jackson (2016). flexsurv: A Platform for Parametric
  Survival Modeling in R. Journal of Statistical Software, 70(8), 1-33.
  doi:10.18637/jss.v070.i08

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{flexsurv}: A Platform for Parametric Survival Modeling in {R}},
    author = {Christopher Jackson},
    journal = {Journal of Statistical Software},
    year = {2016},
    volume = {70},
    number = {8},
    pages = {1--33},
    doi = {10.18637/jss.v070.i08},
  }