Mental health nurse Amanda Young is accused of administering a lethal dose of clozapine to Joshua Gafney, 22, at his home in February 2012.
Bristol Crown Court heard she mistakenly thought each 14ml bottle contained 50mg of clozapine, when in fact this figure was per millilitre.
Ms Young, 40, of Yeovil, denies manslaughter by gross negligence.
Mr Gafney, who had serious mental health issues, died after being given 14 times the amount of clozapine needed to treat him, jurors have been told.
The court heard Ms Young had told police she could not explain what she had done, but had been "working under extreme pressure that day".
In a police interview, she said she had not seen the medication in that form before and although she thought the dosage was large, she had checked with colleagues who agreed it was the right amount.
She told police she had always been very cautious when it came to medication and she "didn't mean to kill him".
The trial continues.
A nurse who gave a patient a lethal overdose of a drug was confused by the labelling, a court has heard.