Middle Primitive Elvish
sag
root. *bitter, poison
maktā-
verb. to fight
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
An unglossed root in The Etymologies of the 1930s with derivatives like ᴹQ. sangwa/N. saew “poison” and ᴹQ. sára/N. saer “bitter” (Ety/SAG). Later poison words were derived from √SLOY, but I think this root is worth retaining in Neo-Eldarin as the basis for “bitter”.