Can a player be at negative life total without losing if a replacement effect prevents loss?

Under CR 704.3a, a player loses the game if their life total is 0 or less. State-based actions check this condition and cause the loss. However, replacement effects (CR 614.1) can modify events before they happen, potentially changing how life loss is handled.

The most prominent example is Phyrexian Unlife: while it's on the battlefield, a player can't lose the game due to having 0 or less life (its first ability replaces the loss condition), and damage dealt to that player causes them to get poison counters instead of lose life once they're at 0 or below. This means a player's life total can go negative without triggering the state-based loss — the loss condition is replaced entirely for that player.

It's important to distinguish prevention from replacement. Prevention effects (CR 615) stop damage or life loss from occurring at all. Replacement effects instead substitute a different outcome. Both can result in a player surviving at 0 or negative life, but through different mechanical means. CR 704.5a is the specific state-based action that checks for 0 or less life.

Concrete example: A player is at 1 life and controls Phyrexian Unlife. An opponent deals 5 damage to them. The player drops to -4 life, but because Phyrexian Unlife replaces the 'lose the game' condition, state-based actions do NOT cause them to lose. Instead, excess damage becomes poison counters. They survive at -4 life until another effect removes Phyrexian Unlife or they accumulate 10 poison counters.

So the answer depends on whether a replacement or prevention effect is actively in place. Without such an effect, reaching 0 or less life causes an immediate loss the next time state-based actions are checked (CR 704.3).