Classic NES Series: The Legend of Zelda Cheats

As soon as you enter Level 1, exit. Then, go back into Level 1. You will notice that the northern door is now unlocked.

When your health gauge is low, visit a lake of a Great Fairy to restore your life. When you approach the lake, your health will slowly fill. While your health is being restored, press start to enter the item selection subscreen, and your health will continue to fill. Using a controller in port 2, press Up + A to return to the save menu. Select SAVE, and load your character's file. Once loaded, instead of remaining at the default of 3 filled heart containers, Link will have his health fully restored at the start.