If a pitcher allows a run which gives the opposing team the lead, but this lead is not held for the remainder of the game, that pitcher does not get the loss. To be the pitcher to have allowed a game's go-ahead run, a pitcher is responsible for the plate appearance which puts the runner on base who scores the run, which is not necessarily the pitcher who is responsible for the plate appearance which causes this runner to score.
For example, on April 13, , Carlos Zambrano of the Chicago Cubs was facing the Cincinnati Reds in the top of the 5th inning. He was taken out of the game with the Cubs leading 5-4 and the bases loaded. The pitcher who replaced him, Will Ohman, proceeded to allow two of the runners on base to score, giving the Reds a 6-5 lead. Although Zambrano was not pitching at the time the runs were scored, he is charged with the loss, as the base runners who scored were his responsibilities.
The pitchers who receive the win and the loss are known, collectively, as the pitchers of record. Every completed non-tie game must have exactly one winning pitcher and losing pitcher each. Pitchers of record can never be awarded a save.