Hearts that are present upon entering a room for the first time are unaffected by the reduction (for example, the ones found in Curse Rooms/Devil Rooms/Angel Rooms), but will be converted to Black Hearts.
The exceptions to this are particular Super Secret Rooms, which have the effect of forcing all heart drops to be the kind found within them.
Things that can drop pickups other than hearts will keep the same chance to drop a consumable, but with an increased chance for drops other than hearts and a decreased chance for heart drops. In particular, this increases the chances of acquiring:
Black Feather: Black Feather grants a damage boost for Daemon's Tail, provided Isaac has an extra trinket slot ( or has gulped/smelted either of the trinkets).
Interactions[ | ]
Crystal Ball: The chance to drop cards or runes will change to 90%/10% in favor of cards.
Tips[ | ]
Due to its specific mechanic, Daemon's Tail has no synergy with items increasing chances for Soul/Eternal/Black Hearts to drop instead of a Red Heart, like Mitre, Mom's Pearl, or Bible Tract.
Since the frequency of heart drops is lowered, holding this trinket can be dangerous to Isaac if he has numerous Red Heart containers, as he can easily find himself short on health.
Mom's Pearl or Bible Tract are trinkets which only need to be held to be useful, whereas Daemon's Tail is more effective if it is held and dropped at the right times, to get an increased chance for consumables from certain mechanics (to counter a lack of Bombs/ Keys/coins, or to get Black Heart protection), and to still get enough heart drops to stay alive at other times (from bosses or familiars).
This trinket nearly requires this kind of strategy to pay off, making it tough to use as the player will have to sort and remember the moments where they must pick up or drop the trinket, as it interacts with nearly every consumable dropping mechanic.
Keys.
The 
Azazel Unlocks 
Satan Unlocks 
The Binding of Isaac: Rebirth
