The Binding of Isaac: Rebirth Item Synergies Guide

Synergies are the heart of The Binding of Isaac: Rebirth. With over 719 items, the number of possible combinations is astronomical, and discovering a broken synergy is one of the game's greatest joys. A synergy occurs when two or more items interact to produce an effect greater than the sum of their parts — or, in some cases, a dangerous anti-synergy that ruins your run. This guide covers the most important synergies, anti-synergies, and transformation interactions in Repentance+.

Brimstone Synergies

Brimstone is the most synergy-rich item in Isaac. It replaces your tears with a chargeable blood laser. Key Brimstone synergies include: Brimstone + Polyphemus (massive piercing beam with damage multiplier), Brimstone + Tammy's Head (room-clearing laser burst), Brimstone + Technology (laser rings while charging), Brimstone + Ipecac (explosive laser — dangerous but powerful), Brimstone + Lump of Coal (damage increases with distance traveled). Anti-synergies: Brimstone + Dr. Fetus (bombs instead of laser), Brimstone + Mom's Knife (knife overrides brimstone in some versions).

Damage Scaling Synergies

• Proptosis + Cricket's Head + Magic Mushroom — Massive close-range damage. Pair with flight or range reduction for melee builds.
• Sacred Heart + Multipliers — Stacks multiplicatively with Cricket's Head, Magic Mushroom, and Eve's Mastery for extreme damage.
• Technology + Technology 2 — Dual lasers with independent targeting. Scales with all tear modifiers.
• Compound Fracture + Haemolacria — Splitting tears that burst on impact. Creates screen-filling damage.
• Trisagion + Piercing items — Persistent beams that hit multiple times per enemy.

Dangerous Anti-Synergies

Some item combinations actively harm your run. Ipecac + My Reflection causes tears to return and explode on you. Dr. Fetus + Epic Fetus replaces manual aiming with random targets. Cursed Eye + items that fire extra tears can trigger long charge cycles at dangerous moments. Technology Zero converts tear effects into a melee shockwave, disabling many ranged synergies. Always evaluate new items against your current build — a quality 4 item can break a carefully constructed synergy.

Transformation Synergies

Collecting three items from a transformation pool triggers a permanent form change. Guppy (cat items) grants multiple free items per room. Beelzebub (fly items) provides a permanent fly familiar with strong contact damage. Leviathan (devil items) gives a permanent devil transformation with damage boost. Seraphim (angel items) grants an angel transformation. Bookworm (book items) provides a permanent tear upgrade. Plan transformations intentionally by picking up related items. See our all items guide for transformation pool membership.

Use our synergy lookup tool to check specific two-item interactions. For item pool context that affects which synergies are available, visit the item pools guide and quality tier list.