A dental nurse broke down in tears as she denied trying to poison her boss.
Ravinder Kaur, 34, is charged with administering a poison or noxious substance with intent after she allegedly put mercury amalgam - a substance used in dental fillings - into a cup of coffee she made for practice manager Laura Knowles.
She is said to have committed the offence at Shams Moopen Dental Practice in Shefford, Bedfordshire, out of "spite or revenge" after receiving warnings about her poor performance at work.
She told a jury of eight men and four women at London's Blackfriars Crown Court that she could not speculate on who was responsible but she had not sabotaged Ms Knowles's coffee.
Breaking down in tears, she recalled being summoned to the office on March 16 last year by her bosses hours after the incident.
"I was just so shocked - this is affecting my whole life, my liberty is at stake," she said.
"They presented the allegation saying I had put mercury in Laura's coffee this morning. They said they wanted to take legal proceedings against me."
The defendant, of Ettrick Drive, Bedford, said she had been "stressed and under a lot of pressure" while being on paid suspension for the last year.
Ms Knowles experienced stomach cramps after drinking the contaminated coffee but did not suffer any long-term damage.
The alleged offence happened three days after a meeting to discuss Kaur's "poor performance" at work. She had received two warnings - one on January 17 that year for "lording it over" other dental nurses and not cleaning, and another on March 13 for leaving a patient in a chair while she went for a break and refusing to work with a dentist.