Substantial amount of information on cancer outcomes is available from RCTs. Hence, making use of such data could save time and spare patients from inclusion in further trials. However, methodological challenges must be addressed to enhance the likelihood of reliable conclusions. Advantages of post-hoc analyses of RCTs include quality of data collected and sometimes randomisation to exposure of interest. Limitations include confounding and sample size, which is fixed to suit the purposes of the trial, insufficient duration of exposure and identification of underlying biological mechanisms relating treatment to cancer to formulate the most appropriate post-hoc study design.