The accused, identified as Reta Mays, was employed at the Veterans Affairs Medical Centre (VAMC) in West Virginia where she committed the crime.
Mays, 46, admitted in federal court that she deliberately injected eight elderly patients with lethal doses of the insulin, which led to the death of seven of them.
She also pleaded guilty to one count of assault with intent to commit murder involving the death of the eighth veteran.
The justice department said in a statement that Mays was working the night shift during the same period of time the veterans died in her care while on admission there.
“Nursing assistants at the VAMC are not qualified or authorised to administer any medication to patients, including insulin.
“Mays admitted to administering insulin to several patients with the intent to cause their deaths,” the statement read in part.
It stated that Mays faced up to life in prison for each count of second-degree murder, and up to 20 years in prison for assault with intent to commit murder.
Under the Federal Sentencing Guidelines, the actual sentence imposed will be based upon the seriousness of the offenses and the prior criminal history, if any, of the defendant.