The fact that he lied envenomed their relationship.

Envenomed was created on 2000-10-17.

Envenom with the build that is in the related links

Main Entry:envenomPart of Speech:verbDefinition:poisonSynonyms:acerbate, alienate, anger, antagonize,contaminate, embitter, empoison, harm, impair,infect, injure, make bitter, make sick, rankle,taint

not actually a word but "toxical"

Or it could be "envenom," a verb meaning to poison. Or maybe "arsenic" or "cyanide" which are poisons.

or baneful, corrupt, hurtful, nocuous, noisome, peccant, and vicious