When low on health, wait until you get to a checkpoint. Save the game at that point. Load the game. This will take you to where you were previously and with a full health bar. This is useful, as it saves on medipacs which you will need later in the game. Alternately, if Lara is low on health and you do get to a checkpoint, kill Lara (for example, jump off a high ledge). This will return you to the previous checkpoint with a full health bar.