A national diabetes charity has issued a warning about a new business offering "potentially catastrophic advice".
An 82-year-old woman's family said she became ill following dietary information from Diabetes.Cymru.
The charity Diabetes UK Cymru said people were also confusing it with the company.
Eryl Vaughan, the director of Diabetes.Cymru, said it has changed the advice on its website and that many organisations have similar names.
Diabetes UK Cymru, which was established in 1934, said it had been 'inundated' with calls from people mistaking their charity with Diabetes.Cymru.
BBC Wales spoke to the relative of an 82-year-old woman who said she was given advice to cut out carbohydrates from her diet at a Diabetes.Cymru stand at this year's National Eisteddfod festival on Anglesey.
The concerned family member, who did not want to be named, said that within a week of taking the advice, the elderly woman was suffering severe stomach cramps.
"We had to call the district nurse to her house due to the pain that she was in," said the relative.
The woman was worried the illness was life threatening, and told her family that she "felt it was the end for her".
The woman, who had been diagnosed with diabetes only a few weeks before being given the advice, has since met with a diabetes nurse and said she has not had any further problems.
Mr Vaughan told BBC Wales that Diabetes.Cymru had not advised anyone to cut carbohydrates from their diets entirely, rather "advised people to limit carbohydrates".
"You're asking me to respond to hearsay and I won't do that. At the end of the day I want to know the detail and if we have clarification to make to the lady or to her family, I'd be very happy to do it," said Mr Vaughan, who has type-2 diabetes himself.
His organisation's website also advised people to stop eating carbohydrates, although this advice has since been updated.
Dai Williams, the national director of Diabetes UK Cymru, said: "The potential impact of the advice can be quite catastrophic because you don't really know what surrounds that person, you don't know what kind of diabetes they've got and what medication they're taking."
He added that a no-carbohydrate diet could make some people seriously ill.
"We all have reputations," said Mr Williams.
"Diabetes UK Cymru has a reputation which we stand or fall by. It does concern me that people are mistaking us for a cowboy outfit that's giving potentially misleading information and we get tarred with that brush.
"We work very closely with healthcare professionals, we wouldn't go out and give the kind of advice that's being bandied about in our name at the moment."
Mr Vaughan, who acknowledged he had no medical qualifications, responded to the name issue: "Many organisations in many fields share similar names. It shouldn't be a problem."
Mr Vaughan is also known for organising the Lap of Wales Challenge, which saw a number of Welsh celebrities undertaking a week-long journey through Wales in 2015 to raise awareness of the Welsh government's changes to the organ donation law and to raise money for Awyr Las/Blue Sky - Betsi Cadwaladr University Health Board's charity.
Auditors later questioned the way the health board handled funding awarded to the charity event.