Needlebite Trap

Instant — Trap

If an opponent gained life this turn, you may pay rather than pay this spell's mana cost.
Target player loses 5 life and you gain 5 life.

2009-10-01:
You may ignore a Trap’s alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.
2009-10-01:
Casting a Trap by paying its alternative cost doesn’t change its mana cost or converted mana cost. The only difference is the cost you actually pay.
2009-10-01:
Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.
2009-10-01:
Needlebite Trap’s alternative cost condition checks only whether life was gained. It doesn’t care whether life was also lost. For example, if an opponent gained 4 life and lost 6 life during the turn, that player will have a lower life total than they started the turn with — but you can still cast Needlebite Trap by paying {B}.