On use, rerolls all items in the room into copies of stackable items that Isaac has.
"Stackable" items only includes passive items, and excludes active items or items that have no useful effect when stacked.
Notes
Approximately 56.8% of all passive items (375 out of the 660 passive items, including 113 added in Fiend Folio: Reheated as of V2.8) are not considered as stackable, and can not be rerolled into by Loaded D6.
If Isaac has no valid items that are considered stackable, then Loaded D6 will reroll items into any of the items Isaac has.
Items cannot be rerolled into "quest" items (i.e The Polaroid and The Negative), nor can quest items be rerolled by Loaded D6 at all.
Video Showcase
Trivia
Loaded D6 is based on loaded dice, dice which are modified to contain a heavy object inside (usually a coin of sorts) to heavily increase the chances of it landing on a certain result.
The Polaroid and 
The Negative), nor can quest items be rerolled by Loaded D6 at all.
