We don't have good data for long term high G tolerance, but estimates I've seen are usually around 1.5 G maximum. However, engine efficiency usually comes at th

We don't have good data for long term high G tolerance, but estimates I've seen are usually around 1.5 G maximum. However, engine efficiency usually comes at the expense of thrust; there are few theoretical designs even that could provide 1.5 G for the amount of time needed to get to Mars. For this engine, for example, the specific impulse is twice that of a chemical rocket - so around 800. At an acceleration of 1.5 G, this engine could only run for around forty minutes before exhausting all the fuel in the vessel. So we are not yet at the technology level necessary for Brachistochrone trajectories (accelerate halfway there, flip over, decelerate the rest of the way); this would merely allow for a shorter Newtonian transfer orbit.