Bob's Rotten Head is an activated item.
A fly-ridden, rotting green head. It appears to have something similar to a maggot in one of the eye sockets. 
Upon activation, Isaac holds the item above his head and throws it in the cardinal direction chosen. Upon detonation, the item acts like an IPECAC shot and poisons enemies. The item can also destroy rocks and, as with any bomb, damage Isaac. The damage for the explosion, direct hit and poison is the same as IPECAC (but the poison lasts much longer here).
- The Carrion Queen takes set damage from the blast (each about one third the strength of a normal bomb) and tear damage a direct hit to the weakpoint, but no poison. Chub takes the noted explosion value per part hit by the blast, gets the poison on each segment of his body and takes the direct hit on whatever part it touched. The division of explosive damage can be stronger than the regular explosion when all three parts are impacted. The poison applied to all three parts adds up to a damage value not far off from a High Priestess stomp. DMG up only affects the direct hit in these cases. The noted applies to Pin, C.H.A.D., Larry Jr. and The Hollow.
- Wrath/Super Wrath doesn't take poison damage, but their explosive damage is set at almost double.
- With Scolex, a direct hit on the weakpoint as it fires upon you will yield normal damage and poison (stopped when it burrows). The head (while firing) can't be hit. The body as it dives takes 5x explosive damage (still altered by DMG) and 5x poison (not ended by burrowing). The normal poison can take at the same time or separately. If you hit the head during the dives it will do nothing. A graphical glitch poison bomb is left where he dies. Leaving the room and coming back reveals a live bomb.
- Wrath/Super Wrath are immune to its poison (as with Bob's Curse). They take special damage from the explosive aspect of Bob's Rotten Head that ignores our DMG (almost as strong as a Mega Bomb).
- Tear damage upgrades will alter the contact and explosive damage, but not the poison. Range will increase range. Shot Speed affects it, but does not appear to affect knockback with it. Tears upgrades allow more rapid use of the item, but this doesn't tend to factor outside an infinite item power scenario.
- Direct contact hits can activate special tear effects which act as you would expect (Spider Bite slows, Mom's Contacts freeze, etc.) in most cases, so detailing all the cases is not necessary.
- Chemical Peel powers up every other use of Bob's Rotten Head.
- Mom's Eye and Loki's Horns can activate, but only produce normal tears.
- The Parasite produces normal tears on split from direct hits.
- The piercing effect will cause Bob's Rotten Head to not explode on contact with an enemy. Spectral tears don't really seem to apply to this item.
- Tough Love alters the explosion and tear damage based on Luck and chance, but doesn't change the appearance of Bob's Rotten Head.
- Chocolate Milk applies by charging (arrows/mouse) before tossing BRH (longer charge for more damage).
- Pinky Eye (slightly stronger than normal poison) and Common Cold (half as potent as Pinky Eye) take the place of the remaining ticks at their reduced damage value. They do get slightly more ticks than Bob's Rotten Head's poison would normally in this case, but their lesser damage takes over. Other poison sources override Bob's Rotten Head and vice versa.
- Familiars will fire when you toss Bob's Rotten Head.
- This item is one of the "dead items" that can be used to unlock The Parasite.
- The boss Sloth was nicknamed 'Bob' by Edward Mcmillen.
- Due to very bad interaction with Brimstone, if you use the Rotten Head while you have Brimstone, you will not be able to charge and shoot Brimstone, drop bombs, and/or use your Q item. You will be stuck in the pre-thrown position and will not be able to shoot or throw anything.
- Dealing poison damage to War can cause him to get stuck off screen for an amount of time.
- Technology 2 is a large Tears Down for use of Bob's Rotten Head by itself, but as Cain you will not be able to use this active as with The Candle.
- Another Activated Item can be grabbed while Bob's Rotten Head is held above Isaac. This will use up that item's charge, but still works after its charge is gone. This can be exploited ad infinitum and the bomb can even be carried between rooms.
- While Bob's Rotten Head is active, other animations are negated such as pill and item pick-ups.
- If Mom's Knife has been thrown and Bob's Rotten Head is activated, it causes the knife to hang in the air until The Candle has been fired or deselected. This interaction, when combined with homing tears (Spoon Bender, Sacred Heart, or The Magician), causes the floating Knife to home in on enemies. Said knife may only hit a given foe once for contact damage.
- The explosion can glitch out on the top and right walls. This most reliably happens after you take a hit and then use the item when pressed against either of the noted walls. It will not harm foes or yourself in this instance.
- Some enemies only take damage from the direct hit (tear), but not the explosion or poison of Bob's Rotten Head. These include: Boil, Sack, Walking Boil, Walking Sack, Gurgle, Mom's Heart/It Lives, Sloth/Super Sloth, Fred, Pestilence, Duke of Flies/The Husk, The Bloat/Peep, Headless Horseman (both parts), Daddy Long Legs/Triachnid and Ultra Pride (Edmund).
- If you are hit with Bob's Rotten Head in a room (without leaving), pick up Best Friend, use it (before the long poison duration finishes) and happen to have red hearts (part of the glitch kill condition), you will take seven hearts of damage per tick (likely killing you).
- The Parasite - Collect two "dead items" (Bob's Rotten Head, Dead Cat, Max's Head, Tammy's Head) in the same playthrough.