Nicholas de la Poer KC, counsel to the inquiry, asked Ms Farmer if that was an unavoidable error.
She replied: “No, if it’s not prescribed I don’t know why it was given.”
Mr de la Poer asked her if this “showed poor insight” by Letby, who replied “perhaps” it was.
Ms Farmer was also asked about a review of the Chester neonatal unit by the Royal College of Paediatrics and Child Health in September 2016, two months after Letby had been removed from clinical duties.
In a statement to the inquiry before her evidence, Ms Farmer said some of the questions by that review had left nurses “upset and tearful”.
When asked why this was, Ms Farmer said: “When Lucy Letby was removed we all felt under suspicion and if it wasn’t Lucy it could be one of us.
“We just didn’t know and I think we felt we hadn’t been supported by the senior managers at that time.
“We felt let down by a lack of communication. We were told we couldn’t speak to our colleagues about it.”
She added they did not get “any support from the doctors” and the situation left them feeling “very stressed and very emotional about it”.
Letby, of Hereford, was convicted of murdering seven babies and attempting to murder seven others, including one she tried to kill twice, and is serving 15 whole life prison sentences.
The inquiry continues.
