Nifedipine slow-release had a half-life of 2 - 5 hours , a mean distribution volume of 6.2 1.9 L/kg ( calculated while using a fixed biological availability of 0.45 taken from the literature due to lack of intravenous data in this population ) compared to a half-life of 6 - 11 hours , and a distribution volume of 1.2 - 1.3 L/kg described in non-pregnant subjects in the literature .