Ferocious Loyalty

 is a perk in Fallout: New Vegas.

Effects
The power of your personality inspires die-hard loyalty from your followers. When you drop below 50% health, your companions temporarily gain much greater resistance to damage.

This perk does not increase your companions damage threshold but rather grants them damage resistance similar to how Med-X or Slasher function for the Courier. The exact amount is unknown, however it is significant enough with high nerve to increase their survivability in hardcore mode.

Ferocious Loyalty will remain active as long as your health is below 50%, so the effect can be thought of as sustainable.