Impact of policy and epistasis on useful therapeutic life. Policy duration is plotted on the Y-axis and the drug usage on the X-axis. The chart compares full epistasis with the ‘mixed’ epistatic mode of resistance for MFT and sequential drug application assuming a fitness penalty of 10%. The useful therapeutic life always lasts longer in clinical/epidemiological settings requiring full epistasis rather than mixed model. (A) Two drugs are available. (B) Three drugs are available.