Too much "sugar-free" chewing gum can lead to severe weight loss and diarrhoea, doctors warn.
The cause is sorbitol, a widely used sweetener in chewing gum and sweets, which acts as a laxative.
Writing in the British Medical Journal, experts gave the example of two patients who had become ill after chewing around 20 sticks of gum a day.
Industry representatives said sorbitol was a safe product though packs carried warnings about excessive consumption.
Sorbitol is widely used in "sugar-free" foods, including products for people with diabetes.
As possible side effects are usually found only within the small print on foods containing sorbitol, consumers may be unaware of its laxative effects and fail to recognise a link with their gastrointestinal problems.
It is also used as a laxative but despite warnings on packets of chewing-gum and other products containing sorbitol, many people do not realise that large amounts will cause stomach and other health related problems, German researchers said.
One 21-year old woman had suffered with diarrhoea and stomach pain for eight months and had undergone a raft of tests before doctors realised her chewing gum habit was to blame.
She lost one and a half stone (11kg) in that time and was underweight.
In a second case a man was admitted to hospital after losing over three stone (22kg) over a year and suffering diarrhoea; they were found to consume between 20 and 30g of sorbitol per day.
Each stick of chewing gum has around 1.25g of the sweetener. Dr Juergen Bauditz, from the Department of Gastroenterology at Charite University Hospital in Berlin, said 5-20g of sorbitol would be enough to cause minor stomach problems such as bloating and cramps but more than 20g could cause diarrhoea and, as these cases showed, severe weight loss.
When he questioned the patients, he found they had replaced the gum sticks frequently, accounting for the high doses of sorbitol which were getting into their system.
Once the patients cut out sorbitol from their diet, their symptoms disappeared and they put on the weight they had lost. "As possible side effects are usually found only within the small print on foods containing sorbitol, consumers may be unaware of its laxative effects and fail to recognise a link with their gastrointestinal problems," he said. "The investigation of unexplained weight loss should include detailed dietary history with regard to foods containing sorbitol."
Sorbitol occurs naturally in a wide variety of fruits and berries including pears, plums, cherries, dates, apricots, peaches and apples.
It is well documented in medical literature, with studies going back more than 20 years, that excessive consumption of polyols, such as sorbitol, can have a laxative effect in some individuals."
The safety of sorbitol has been thoroughly reviewed by health and regulatory bodies, including the WHO/FAO Joint Expert Committee on Food Additives.