Antimony

TL3; $1; LC3.
Antimony has effects quite similar to those of Arsenic. It is usually administered as tartar emetic, a medicinal substance sold in the Victorian era and used, among other things, as a sedative. It has a slight bitter taste (-3 to Taste rolls, see p. B437 regarding Taste rolls for poisons). Holdout rolls are at +2; Sleight of Hand rolls are unmodified.

Source: GURPS 4th - Mysteries, page 36