Room drops: For calculating room drops, luck is capped between 0 and 10, with higher luck not only resulting in higher amounts of drops but also higher quality drops such as chests.
Room drop rates are otherwise identical in Normal and Hard game modes, but Hard mode removes 66%/ 33% of heart-type drops, replacing them with nothing.
If there is more than one playable character on screen (Jacob and Esau, Co-op, using the item Strawman, etc.) the room drop rate depends on the total luck the 2 characters have.
Tear effect chance: The chance of shooting tears with status effects becomes more likely as Luck goes up. This scales up to 100% chance per tear in most cases with enough Luck.
Most of the status inflicting effects require high Luck to activate on every tear.
Since these effects persist on the enemies for a set amount of time regardless of the Tears stat, a fast rate of fire is often enough to affect an enemy indefinitely.
Most tear effect chances scale exponentially with luck, while a few others scale linearly with luck, meaning that it can take a lot of luck to notice a significant increase in activation rates in most cases.
However not all effects scale up to 100%, and some items have the rates capped at a lower percentage. If the maximum activation rate is not 100% for an item, the true maximum rate will be listed instead.
+1 luck. Improves Isaac's chances at various random events (independent from the luck stat). Converts all bad pills into good pills, without identifying them.
Reveals the location of the Boss Room. Upon defeating a boss, activates XIX - The Sun, fully recharges Isaac's active item, grants bonus damage and luck for the rest of the floor, and removes curses.