The representative organisation for people with Spinal Muscular Atrophy in Ireland have described the HSE's criteria to access Spinraza as "crude".
Following months of negotiations, the Health Service Executive yesterday announced that the drug to treat SMA had been approved.
However it will only be available to people under the age of 18, of which there are 25 in Ireland.
In a statement, SMA Ireland has said it accepts there must be some criteria for access to the life-changing drug.
However it describes the "arbitrary age cap" as "a crude method with little scientific support".
It points out that formal adult trials for Spinraza have only started and will not be completed until 2022 or 2023 at the earliest.
"If it is the HSE position that treatment cannot be considered without this data, this may be too late for some of our adults with SMA," it says.
It is estimated that 25 adults are living with SMA in Ireland.
The Chairman of SMA Ireland Anton Mannering has questioned how excluding those over 18 is justifiable when treatment will be given to a teenagers aged 17 until later in life.
"It is intolerably cruel and discriminatory to inflict that type of injustice on a person suffering from a degenerative disease," he said.
"Especially when the only difference between a treated and an untreated individual might be which side of voting eligibility they might fall."