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Elizabeth Perkins, 30, suffered a severe allergic reaction and fell into a coma for three days after taking just one sip of the diet drink.
This is because Elizabeth and her two sons are severely allergic to artificial sweeteners aspartame and asphetame - found in thousands of products, but used in high quantities in diet soft drinks.
And now she's speaking for the first time after her rare condition nearly killed her when a member of staff at her local pub served her a diet drink when she'd asked for a full-fat one - which contains sugar rather than sweetener.
Elizabeth, from Swadlincote, Derbyshire, said: "I almost died after drinking diet cola instead of the full fat version I had asked for. I don't think people realise what a big difference something like that makes, and since the sugar tax, fewer and fewer places are serving the full fat drinks.
"Some people automatically give you the low-sugar or sugar free versions but for me, that mistake could have been fatal. I'd asked for a full fat coke, and stressed that it had to be full fat as usual to the bartender.
"But once I took my first sip and straight away, I knew it tasted strange, and I could feel the usual sickness start, like I was going to be sick. I instantly felt dizzy and felt the room go dark as my body tried to fight off the allergic reaction.
"I learnt later that a friend had guided me into a chair but I'd just passed out. I had less than a sip of diet cola I was rushed to hospital, in a coma for three days. And it had taken three days for it to work its way out of my system - it was terrifying."
Elizabeth has had to avoid artificial sweeteners for her entire life - and the condition which causes the allergy is genetic. Her two sons are also at risk of being seriously ill every time they reach for a sweet treat.
Elizabeth claims the sugar tax has made her family's life hell, as she has now has to make everything - from her own yogurt to her own squash.
She has learnt to manage the condition over the years - but it's meant that everything that goes in to Matthew, six, and two-year-old Jacob's lunchboxes is entirely home-made as Elizabeth has found almost everything contains the deadly ingredients.
She added: "Since the sugar tax came into effect, everything seems to contain the dreaded artificial sugars. When I asked in the bar, I made sure to make myself explicit but the problem is most people don't seem to see the difference.
"To me, they taste different but it seems not everyone realises that. Since the tax came into effect, it's not cost effective or easy for pubs to have the sugared versions, as the prices have to be put through the till differently.
"Now, it's just easier for me and my sons to just forgo the social outings, so we know exactly what we're getting at home."
And despite constantly scouring all food packaging, little mistakes can prove almost fatal.
Elizabeth went on to explain: "We don't have the enzymes to break down the sweeteners, so it is like instant poison and we will immediately start vomiting and becoming dizzy if we have it.
"In extreme cases, we could end up in a coma. But both my boys get put in life threatening situations. Jacob was diagnosed with a benign tumour and needs medication, and a few months Matthew suffered a water infection.
"They were prescribed medication but I was shocked - most places don't supply the sugared versions as it's no longer cost effective for the manufacturers.
"I had to drive around for hours to get Matthew the medication he needed, whilst he was shaking violently and had a temperature of 41°C. It breaks my heart, especially for the boys.
"They can't even have birthday cakes like their friends because the icing contains the artificial sugars. It's so time consuming to make my own of everything, I have to scour all packaging and I have to make cakes and biscuits from scratch.
"I can't just get them off the shelves. I check and double check everything, but for Matthew's birthday, at the last minute I was told I would have provide the meals for my boys, whilst their friends enjoyed the party food.
"It feels like we're penalised, for needing the sugared versions of things when really, it's a necessity - we need it to survive."
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