Libra 
Effects
Notes
- The balancing is based on Isaac as the character himself; he gains no bonuses or penalties to any stats from simply having Libra on its own.
- The game calculates Isaac's speed, range, tear delay, and damage before using them in Libra's balancing formula. This has the following notable effects:
- (Removed in Repentance) If Isaac's Speed or Tear delay are already at their internal soft caps (2.00 and 5 respectively), items that increase them specifically will give no stats.
- The only exception would be items that decrease tear delay, and then only if Isaac's tear delay isn't at the hard cap of 1.
- (Added in Repentance) If Isaac's tears stat has reached the soft cap corresponding to 5 fire rate, basic tears ups no longer add bonuses. Likewise, direct fire rate increases have no effect if the absolute maximum of 120 has been reached.
- Speed adjustments, despite the stat being bounded between 0.1 and 2.0, always have an effect.
- Items that apply multipliers to Isaac's tear delay (
Mutant Spider, 
Brimstone, 
Dr. Fetus, etc.) can be fired more rapidly with less damage.
- Damage upgrades become less effective. In return, speed and range upgrades now increase damage and tears.
- Items that affect the stats of Isaac's tears after they're fired (
A Lump of Coal, 
Blood Clot, 
Proptosis, etc.) do not go through the balancing formula and work as normal.
- (Removed in Repentance) Libra's balancing formula values tear delay higher than the other stats and damage less than the other stats. This makes standard tears ups give more stats than a standard range or speed up, and standard damage ups give less stats than a standard range or speed up.
- (Added in Repentance) Generally speaking, if an item increases Isaac's damage per second, Isaac will receive a stat bonus. If the item lowers fire rate and increases damage, or vice versa, in a way that results in loss of DPS, Isaac's stats will go down.
- (Removed in Repentance) Since tear height is not involved in the calculations, items such as
Mini Mush or (Added in Afterbirth)
Kidney Stone that increase tear height but lower range are mostly detrimental.
Synergies
Soy Milk: As tear delay is valued very highly by the formula, greatly increases all stats.
- (Added in Repentance) Soy Milk is only a minor DPS upgrade, and as such a very minor all stats up.
Ipecac: Can be rapidly fired with decreased damage.- (Added in Repentance)
Rock Bottom: Stats will increase as a result of items, get locked at the highest point, and then be balanced out via Libra allowing items like Ipecac and Soy Milk to have higher damage/shot speed respectively.
Taurus: Gradually increases all stats in an incomplete room. Libra prevents Isaac from becoming invincible (unless with high stats), and stat increases will remain until the end of the fight.- (Added in Afterbirth)(Tape Worm) Tape Worm: As range is valued very highly by the formula, greatly increases all stats. Synergizes well with (Added in Afterbirth)
Mom's Box.
- (Added in Repentance) Tape Worm is a flat +3 range instead of x2, resulting in only a small all stats up.
Interactions
- (Added in Repentance)
Almond Milk: Almond Milk is only a minor DPS upgrade, and as such a very minor all stats up.
- (Removed in Repentance)
Cricket's Body: Due to how Libra values tear delay, Cricket's Body gives an overall stat downgrade or upgrade depending on Isaac's internal base tear delay: At 7 or less it increases stats, at 8 or more it decreases stats.
- (Added in Afterbirth)
D8: D8's stat changes depend on which stats end up with which multipliers and their base values. Range has a high weight and is uncapped, and thus can result in a massive boost if it scores a high multiplier. Damage is also uncapped but, on the other hand, has a low weight.
- (Added in Repentance) The changes in Libra's formula mean D8 is very likely to give a small all stats up, its magnitude depending on Isaac's other items and which stats were emphasized.
- (Added in Repentance)
Dark Arts: Libra is extremely detrimental to how Dark Arts works, and should not be used together: The speed boost during the invincibility phase is transferred into attack bonuses (Which are useless as tears cannot be used), and afterwards, the temporary damage boost is divided across all of Isaac's stats, resulting in very negligible stat boosts.
- (Added in Afterbirth)
Epiphora: The stat bonus for continuously firing in one direction applies to all stats. The accuracy penalty is unaffected.
Eve's Mascara: Gives an overall stat downgrade, due to tears being weighted heavier than damage.
- (Added in Repentance) Eve's Mascara is a DPS up with no other relevant stats, and as such is a small all stats up.
Experimental Treatment: The greater number of stat upgrades compared to stat downgrades results in an effectively guaranteed all stats up. Health, Shot Speed, and Luck are still subject to chance.- (Added in Afterbirth)
Purity: At base stats, the tear delay and damage aura give a greater stat bonus than the range or speed aura.
- (Added in Repentance) All the auras grant a small all stats up of similar magnitudes.
- (Added in Afterbirth)
Succubus: When in the aura, affects re-balanced stats rather than just damage.
- (Removed in Repentance)
Technology 2: Doubled tear delay due to firing out of only one eye is unaffected by the balancing formula.
The Book of Belial.