Two pharmacists are facing claims at a fitness to practise inquiry relating to the supply of a double dose of a powerful drug to a young man who was a minor at the time.

Ana Eusebio and Patrick Staunton, from Staunton's Pharmacy Ltd, Link Pharmacy, on Hopkins Road, Castlebar, Co Mayo, are the subject of the inquiry, which is taking place at the Pharmaceutical Society of Ireland's headquarters in Dublin.

Ms Eusebio and Mr Staunton both face allegations of professional misconduct and poor professional performance.

Supervising pharmacist Ms Eusebio is accused of supplying twice the prescribed dose of Enbrel to the young man, referred to as Patient A, based on prescriptions from October 2011 and March 2012.

Ms Eusebio is also accused of failing to take adequate steps after discovering the error on 17 September 2012.

Superintendent pharmacist Mr Staunton is accused, among other claims, of failing to take adequate steps to ensure the health and wellbeing of Patient A after he found out about the matter on 21 November 2012.

Enbrel is a medication used to treat patients suffering from rheumatoid arthritis and other chronic autoimmune conditions, and is often taken in the form of an injection.

It is managed on a patient-specific basis, and requires a high level of supervision, as it has a number of serious potential side effects.

Enbrel can leave patients at risk of serious infection and, in some cases, can also increase the risk of certain types of cancer.

During his evidence this morning, Mr Staunton said he qualified as a pharmacist in 1982, and is now the superintendent pharmacist for his family pharmacies in Castlebar.

He said the parents of Patient A informed him of the supply error by letter dated 21 November 2012, approximately nine weeks after the last supply error occurred.

"That sent alarm bells and shock waves around the pharmacy," said Mr Staunton.

He spoke with Ms Eusebio, and with Pfizer, which manufactures Enbrel.

Mr Staunton then wrote a letter to the parents of Patient A, who is no longer a minor.

He added that while he had no further contact with the family, the incident concerned him greatly.

"In more than 30 years, this was the first incident of this kind that had happened, and I was deeply concerned about it," he said.

The inquiry continues this afternoon.