The pills, which are readily available from high-street retailers claim to aid weight loss, often by speeding up metabolism.
But experts from the British Dietetic Association have said that there is no evidence that they actually work and urged anyone trying to lose weight to consult health professionals.
A spokeswoman for the association said: “There is no evidence that any ‘diet pills’ have any scientific basis or will contribute to weight loss.
“However, we are aware that many high street supplements available are unsafe.
“Any weight loss seen would be more likely to be attributed to a restricted calorie diet not the supplements.”
The warning comes after Taylor Hannah said she suffered a ripped stomach lining and chronic vomiting from taking herbal diet pills.
Taylor, 19, turned to the pills after medication for her bipolar condition saw her weight balloon from 10st to 14st.
She decided to purchase green tea extracts and fat metabolisers from a high street retailer last August.
Taylor used them for six months until she had to be rushed to hospital and put on a drip because of severe dehydration.
Now, the teenager has lost almost half her body weight and still struggles with eating.
Taylor, from Greenock in Scotland, said: “I almost died from dehydration and needed to be put on a fluid drip due to excessive vomiting. I was really ill, couldn’t move and had chest pains.
“I have now found out they have torn my stomach lining.”
The petite 5ft 3in teen lost almost half of her body weight since taking the pills and being unwell, dropping from 14 stone to a tiny 7st 9lb.
She was admitted to hospital on February 5, and said the whole experience was terrifying.
She said: “I thought I was going to die. I was constantly being sick and if I hadn’t gone to hospital when I did, I would have died because I was so dehydrated.
“No amount of water would fix it. I was drinking up to six or seven litres of water a day and nothing helped. Doctors told me the caffeine from the diet pills could have caused the dehydration.”
Taylor later discovered that one green tea tablet is the equivalent of five cups of green tea.
Taylor said: “I researched diet pills loads before I actually bought them.
“Then when I went into the shop the girl recommended these two in particular, telling me they were the best.”
Taylor bought the pills - Nature’s Garden green tea extract and Nutrition HQ’s fat metabolisers - from high street retailer Holland and Barrett.
The green tea pills, retailing for £19.99, claim to boost metabolism which assists with weight loss.
The red metabolism capsules, which cost £12.99, promote a healthy weight loss, burning fat into muscle.
But Taylor now believes it is these pills that made her unwell.
She added: “The GP said it could possibly be the diet pills, and another doctor said he has also had reports that they can rip your stomach lining.”
Still in chronic pain, Taylor is only able to eat yoghurt and drink milk.
She said: “I’m struggling to eat without vomiting and sometimes I can’t even drink water. I get a burning sensation and then I’m sick and then get severe stomach cramps.
“I’m still not sleeping because I’m in so much pain.”
Taylor now feels angry with herself for ignoring friends’ warnings and buying into the diet pill fad.
She said: “I feel ridiculous for taking them now. Everybody told me not to.
“I fell into the trap of feeling pressured to look a particular way, and took a shortcut to achieve it. I regret ever taking the pills.
“I want people to know that just because pills may be herbal doesn’t mean they are safe.”
A spokesperson for Holland & Barrett said: “All our products are manufactured to the highest standards, used by tens of thousands of people every day.
“We have never received a complaint of this kind from these products before.
“Certain products we sell, including weight management supplements, do carry an on-pack warning instructing customers to consult their GP if, for example, they are already taking prescribed medicines.
“Clearly we are concerned to hear of any customer reportedly becoming unwell, and we would always urge customers to seek advice from our team of nutritionists or trained store staff.”
The British Dietetic Association spokeswoman added: “A high caffeine intake alongside a calorie restricted diet could affect stomach health, as seen here, and cause anxiety, stress (i.e. chest pains).
“Holland and Barrett provide brief training to their staff - mostly on how to sell their products -, however it needs to be clear that they are not medically trained.
“Any person can call themselves a nutritionist. I encourage people to only take advice from a Registered Dietitian or Registered Nutritionist.”