A man with a rare inability to digest protein has said eating a normal diet would leave him brain damaged.
Mark Edwards, 35, has phenylketonuria (PKU), meaning he can only eat 6g (0.2oz) of protein per day - about one egg or two tablespoons of beans.
Eating more than his body could process could cause him to suffer depression, anxiety or even brain damage.
A form of treatment, Kuvan, is not available on the NHS - which instead recommends a strict diet.
A number of MPs are to UK patients.
BioMarin said the NHS had not accepted its "very competitive" offer.
The National Institute for Health and Care Excellence (NICE), which advises the NHS, said it had not begun its appraisal of Kuvan to treat PKU, but has made the "exceptional decision" to ask a panel to look at the issue.
After this, a formal referral will be required from the UK government's health minister to determine the process under which Kuvan will be assessed, a spokeswoman said.
Mr Edwards, from Llanegryn in Gwynedd, was diagnosed with PKU at birth after it showed up in the standard blood tests given to babies - the condition affects one in 10,000 people in the UK.
He spends about £4,000 a year on Kuvan tablets, which allow him to eat more protein - and said they could change the lives of people with PKU.
Amino acids are the building blocks of protein and are broken down by the body to make our own proteins.
But people with PKU cannot properly digest the amino acid phenylalanine, and the levels build up in the bloodstream and the brain - leading to a number of health problems.
"I can't have any meat, no egg, so nothing. Just what is available on prescription, really," said Mr Edwards
With Kuvan, Mr Edwards is able to eat up to 15g (0.5oz) a day, which he called "a massive difference".
"It's been available for 10 years but BioMarin, the company which makes it, they've put a price on it which is too expensive for the NHS," he said.
Mr Edwards has met Westminster politicians to discuss how sufferers of conditions such as PKU have been "massively failed by the system".
He is being supported by Plaid Cymru's Westminster spokeswoman, Liz Saville-Roberts, who said any drug with the potential to improve the lives of those with PKU should be easily and freely available.
Kuvan reduces the levels of phenylalanine in many with PKU.
BioMarin said it had put the drug forward for commissioning five times in the past 10 years with no success.
"The burden and severity of PKU as a disease in the UK is not recognised by NICE or the NHS," it said.
"BioMarin has made an offer to the NHS which is very competitive compared to other markets but it has not been accepted."