A nurse only realised shortly after administering a woman the Covid vaccine that the dose contained six times the required amount.
The 23-year-old recipient, a psychology intern at Noa hospital in Tuscany, central Italy, received the half dozen doses on Sunday (local time) according to CNN.
A spokesperson for the hospital, Daniella Gianelli, revealed the woman, who had no underlying health issues, had been discharged after being monitored for 24 hours.
Ms Gianelli said she was in "good health" after a health worker mistakenly filled a syringe with a whole bottle of the Pfizer BioNTech vaccine, which is equivalent to six doses.
They didn't realise their mistake until after the vaccine had been injected, Ms Gianelli told the publication.
“She saw five empty syringes and realised her mistake,” Ms Gianelli said.
The woman's condition would continue to be monitored following the dose, the spokesperson added.
She was eligible for the vaccine because of her position at the hospital, with the Italian government introducing mandatory Covid vaccinations for healthcare and pharmacy workers.
While an internal investigation would be opened, Ms Gianelli said it was "maybe just human error, definitively not on purpose".
Healthcare workers who refuse to be vaccinated against Covid-19 will be assigned roles that don't have patient contact.
If there are no such positions available, they could face being suspended without pay.
It is still unknown if healthcare workers can face criminal penalties by refusing the vaccine.
Italy's Covid cases have begun to decline slowly after the country had some of the highest numbers in Europe.
Earlier this year, two elderly people in Queensland were given up to four times the required amount of a Covid vaccination.
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