"This question is addressed by the comparison between the two Ipilimumab arms (Ipi monotherapy vs Ipi+gp100), where the hazard ratio was 1.04. If gp100 were significantly toxic and thereby reducing survival in the combination arm, the survival difference between the combo arm and the mono arm would not have turned out to be minuscule."
A 3 arm trial of a toxin and its antidote could show the same pattern.