From A Wiki of Ice and Fire
Sourleaf is a foul tasting plant that's leaves are chewed in a similar fashion to chewing tobacco. It is generally transported in bales. Chewing it causes a pinkish froth to form on the lips, and later unwanted juices are spat out. Chewing sourleaf stains one's mouth and turns the teeth blood red, causing people who chew sourleaf to exhibit a "red smile". Catelyn Stark considered Masha Heddle’s smile a bloody horror. Chewing sourleaf can dull pain.
Characters who chew sourleaf
- Ser Bennis of the Brown Shield
- Ser Emmon Frey 
- Masha Heddle
- Archmaester Marwyn
- The pious dwarf
- Arya Stark
- ↑ A Clash of Kings, Chapter 63, Daenerys V.
- ↑ 2.0 2.1 2.2 A Clash of Kings, Chapter 1, Arya I.
- ↑ A Game of Thrones, Chapter 28, Catelyn V.
- ↑ The Sworn Sword.
- ↑ A Storm of Swords, Prologue.
- ↑ A Feast for Crows, Chapter 33, Jaime V.