A patient died after drinking an overdose of six bottles of medication, even though he was prescribed less than a teaspoon at a time, a report has found.
Joshua Gafney, 22, who should have received less than a third of a bottle, had 21 times the prescribed amount of the drug clozapine.
He died at home in Yeovil shortly after the liquid was administered, allegedly in the presence of mental health nurses Amanda Young and Petia Gummer.
A post-mortem examination and toxicology report found Joshua died from "acute clozapine toxicology" after being given 84ml instead of just 4ml.
The nurses were arrested and charged with Joshua's manslaughter but police have formally dropped the allegation.
Mr Gafney's devastated mother, Tina Marren, also of Yeovil, said she was "shattered" by the decision.
She said: "My son was my life and that was taken away. I will never give up until I get answers for Joshua.
"I feel the authorities are treating Joshua as if he was not important. As a family we are so angered, hurt, and devastated."
Mr Gafney, who was being treated for mental illness psychosis, died at his home after being visited by Miss Young and Mrs Gummer on February 8 last year.
The nurses, who work out of Summerland Hospital in Yeovil, have been suspended by their employer Somerset Partnership NHS Foundation Trust.
They are subject to separate investigations by the trust and regulator the Nursing and Midwifery Council, who have placed restrictions on them.
A report of a condition of practice hearing for Mrs Gummer said: "The panel noted that Mrs Gummer denies the allegations.
"However, having considered the serious nature of the allegations, which involved a gross inaccuracy in drug administration, the panel considered that there is a real risk of repetition of the alleged behaviour should Mrs Gummer continue to practise without restriction."
A spokeswoman for the Nursing and Midwifery Council said investigations would continue and a panel would decide whether the nurses had a case to answer.
The Somerset Partnership NHS Foundation Trust, meanwhile, said it would conduct its own internal investigation following the decision not to bring criminal charges.
An inquest into Joshua's death has been opened and adjourned by East Somerset Coroner Tony Williams. His family are now preparing their case for the coroner along with a solicitor and say they will fight for answers to Joshua's death.
Mr Gafney's inquest is due to be reconvened later this year.
Both nurses declined to comment through the Somerset Partnership NHS Foundation Trust.