Hard mode

Hard mode changes the gameplay compared to Normal in some ways to make it more difficult. When playing in Hard mode, a small red pentagram appears on the HUD, below the pickups.

Differences

 * 2-3 more rooms generate each floor.
 * Standard rooms with a difficulty of "1" cannot generate.
 * Standard rooms with a difficulty of "1" have a reduced chance of appearing.
 * Most rooms with this difficulty are very easy, making difficult rooms more common.
 * Enemies have a base 20% chance to become Champions instead of only 5%; additionally, they only award pickups 33% of the time.
 * Hearts are 66% less likely to drop after completing a room.
 * Hearts are 33% less likely to drop after completing a room.
 * Only champion bosses will drop hearts; normal bosses will not.
 * Floors are more likely to have curses.
 * Blood Donation Machines only give out one coin per use.
 * Also applies to IV Bag.
 * Beggars require a minimum of 6 donations before paying out.
 * This doesn't apply to Bomb Bums or Key Masters.
 * Lower levels of shop can appear even if the store upgrades have been unlocked.
 * The drop rate of soul heart from blue fires is 10%, while it's 25% in normal mode.
 * Certain enemies and bosses have increased shot and movement speed.
 * does not provide the effect of.

Trivia

 * Before update 1.03, the reduced drop-rate affected all pickups (not just Red Hearts).
 * For update 4.00 hard mode also reduced the amount of tinted rocks appearing. This was removed with 4.01 a few days later.
 * Most Hard mode unlocks are co-op babies, with the notable exceptions of, , , , and the character.
 * 's unlock requirements were extended in later DLCs due to the addition of more completion marks, but it can still be unlocked in the base game using the same method.
 * None of the tainted characters have unlocks tied to getting all completion marks, apart from.