With a sample size of 134 patients we will have over 80% power to detect a 50% relative risk reduction in the primary outcome event rate and a large enough sample to determine the feasibility objectives of the pilot study (i.e. obtain a precise estimate of the primary outcome event rate [DVT], a precise estimate of the multicentre recruitment rate, feasibility and acceptability of blinded study drug and placebo administration, feasibility of obtaining local study centre MRV and central interpretation of MRV and a preliminary relative risk reduction estimate with study drug compared to placebo to inform final study sample size determination).