Rule 119.4 · Life
If a cost or effect allows a player to pay an amount of life greater than 0, the player may do so only if their life total is greater than or equal to the amount of the payment. If a player pays life, the payment is subtracted from their life total; in other words, the player loses that much life.
Subrules
If a cost or effect allows a player to pay an amount of life greater than 0 in a Two-Headed Giant game, the player may do so only if their team’s life total is greater than or equal to the total amount of life both team members are paying for that cost or effect. If a player pays life, the payment is subtracted from their team’s life total.
Players can always pay 0 life, no matter what their (or their team’s) life total is, and even if an effect says players can’t pay life.