Each tear will do Isaac's current damage and a flat +25 damage bonus on top.
Notes
The effect of Tammy's Head is similar to Isaac's Tears, but Tammy's Head will benefit from tear-replacing items like Brimstone, unlike Isaac's Tears.
Synergies
Azazel / Brimstone: Isaac will fire Brimstone lasers in all directions. Each laser is given bonus damage.
This will deal a substantial amount of damage near the center of the effect due to how lasers overlap the closer you get to Isaac.
Book of Virtues: Spawns pale wisps in the outer ring. The wisps' tears deal 1.5 damage, but fire twice as fast as standard wisps. Tears are inaccurate and fire in a spread.
Car Battery: Shoots 20 tears in a circle around Isaac.
Chocolate Milk: The size and damage of tears shot depends on how far Chocolate Milk is charged.
Tainted Azazel: Although Tainted Azazel's Brimstone beams do not have a damage bonus, they also do not have the normal damage penalty and still deal double damage to enemies that have been sneezed on.
Interactions
Dr. Fetus / Ipecac: Tears are replaced by bombs or exploding shots, which may result in self-damage.
Eye of the Occult: The tears quickly lose their outward momentum and can now be controlled all at once, making it easy to make the tears hit enemies.
Mom's Knife: The tears are replaced with knives that deal double Isaac's damage, but move very quickly, have a small hitbox, and don't return to Isaac.
Trisagion: Shoots Trisagion blasts without any increase to their damage.
When Repentance released, this item was removed from Golden Chests, but it wasn't added into any other item pool besides the Crane Game, effectively making it impossible to find normally. This was later rectified.
Bugs
Bug!
With the Mom's Knife synergy, the Blindfolded condition prevents Tammy's Head from shooting knives and makes the item useless.
Tammy's Head is similar to 
Isaac's Tears, but Tammy's Head will benefit from tear-replacing items like 
Brimstone, unlike Isaac's Tears.
Azazel / 
Each laser is given bonus damage.
Book of Virtues: Spawns pale wisps in the outer ring. The wisps' tears deal 1.5 damage, but fire twice as fast as standard wisps. Tears are inaccurate and fire in a spread.
Car Battery: Shoots 20 tears in a circle around Isaac.
Chocolate Milk: The size and damage of tears shot depends on how far Chocolate Milk is charged.
Tainted Azazel: Although Tainted Azazel's Brimstone beams do not have a damage bonus, they also do not have the normal damage penalty and still deal double damage to enemies that have been sneezed on.
Dr. Fetus / 
Ipecac: Tears are replaced by bombs or exploding shots, which may result in self-damage.
Epic Fetus: Fires only normal tears.
Eye of the Occult: The tears quickly lose their outward momentum and can now be controlled all at once, making it easy to make the tears hit enemies.
Mom's Knife: The tears are replaced with knives that deal double Isaac's damage, but move very quickly, have a small hitbox, and don't return to Isaac.
Trisagion: Shoots Trisagion blasts without any increase to their damage.
The Parasite - Pick up any 2 of the following items in a single run:
Bob's Rotten Head, 
Dead Cat, 
Cricket's Head, and 
3DS The Icon of Tammy's Head as seen on the lower 3DS screen.
Crane Game, effectively making it impossible to find normally. This was later rectified.
Bug!
Pet Items 
The Binding of Isaac: Rebirth
