The HSE has said it is assessing the cost of providing two cancer drugs that doctors say provide effective treatment against the disease.
The supply of one drug, known as Pembro, has already ended with a similar access programme for the second drug due to end next week.
Oncologists have said the drugs have had remarkable success in treating some cancers.
The drugs have been provided to around 100 cancer patients free of charge over the past year on what is known as a compassionate basis, while its manufacturer waits to hear if it has been approved for use by the HSE.
Experts have said it has had remarkable success in treating cancers including melanoma, kidney tumours and certain types of lung cancer.
However, leading consultant oncologist Professor John Crown has warned that a delay in approving the drug could have serious consequences for the health of patients currently using it.
He said he believes it is a result of bureaucracy and called on the Minister for Health Simon Harris to take the action needed for the drugs to be made available to patients.
Speaking on RTÉ's Morning Ireland, Prof Crown described the drugs as "game changers" in the field of cancer treatment and accused the HSE of obfuscation over the delay.
He said the medical, scientific and safety assessments have been done and a recommendation was made by the National Centre for Pharmacoeconomics that the drugs should be paid for.
Prof Crown said there was an assumption that the drugs would come on stream sometime during 2016 and there are a small number of patients who have been waiting for these drugs and who are running out of time.
The HSE said that Pembrolizumab has been assessed as cost effective but that its budgetary impact is now being considered as it could cost €63 million over five years.
It said the second drug Nivolizumab has been assessed as not cost effective and could have budgetary impacts of more than €40 million over five years.
In a statement released this evening, the HSE said: "The HSE has a responsibility to source the most effective medicine on behalf of patients at an affordable price to the taxpayer.
"As is the case for all new medicines, the clinical benefits of Pembrolizumab and Nivolizumab are being carefully considered under a process of health technology assessment, in order to determine value for money and patient benefits.
"In the 2016 HSE Service Plan an additional €7m was allocated for cancer drugs to support the National Cancer Control Programme's Systemic Therapy Programme.
"In this context Pembrolizumab, for example, has an estimated net five-year cost implication to the State of €64m at the price being sought by the pharmaceutical company.
"The single biggest barrier to new drugs approvals in Ireland is the high prices that manufacturers are seeking to charge.
"Lower prices would enable the HSE to afford more innovative drugs for more patients as they become available and the HSE will continue to engage with manufacturers in the best interests of patients and the taxpayer," added the statement.
Harris 'concerned' for cancer patients
Minister for Health Simon Harris has said that he is very concerned for the patients affected. In a statement, he said the drugs are being considered by the HSE, and said that the assessment process is "ongoing,"
Earlier, Fianna Fáil health spokesperson Billy Kelleher called on Minister Harris to instruct the HSE to allow immediate access to the new drugs.
Mr Kelleher said the drugs in question were proven to be both cost effective and helpful in the treatment and survival of cancer patients.
He said it was wrong that the office in charge of accessing drugs for use in public care had "given the go ahead for one of the drugs, Pembro, but yet the HSE has dragged its feet in allowing for its use."
Meanwhile, a consultant oncologist at the Midland Regional Hospital Tullamore has said that the supply of the drugs may end next week if the HSE decides not to fund their use.
Speaking on RTÉ’s Sean O’Rourke, Dr Kyran Bulger, said some of his patients have had a "marvellous" response to the drugs, which act by spurring the immune system to attack the cancerous cells.
He said the drugs have had remarkable success in treating melanomas, kidney tumours, non-small cell lung cancer - a leading cause of cancer deaths in Ireland - and some colon cancers.
Dr Bulger said he prescribed the drug for two patients where all other options had been exhausted.
"I gave them this drug in desperation and - I'm only several months using the drugs, but the tumour - we can't see it anymore, the patient's life is back fully functional and that sort of experience, well for me it's spectacular."