A brass knuckle.
Isaac has a chance to fire a tooth instead of a tear, which deals 3.2x damage.
The probability of shooting a tooth is determined by the Luck stat. It is a 1/(10 minus Isaac's Luck stat) chance, thus, it takes nine luck up Pills to be guaranteed to fire a tooth. Luck down will make teeth less likely to be fired since they cause the denominator to increase instead of decrease.
- Mutant Spider, Loki's Horns or The Inner Eye - More than one shot can be a tooth.
- The Parasite - Split shots can also be teeth.
- "Tough love" is usually a reference to domestic abuse, particularly regular beatings to children, which corresponds to the grim theme of the game.
- The damage multiplier (3.2) is likely a reference to the fact that the average person has 32 teeth.