Eating disorder charity Beat has called for a change in regulations surrounding the sale of laxatives, following a 30% increase in calls from people using them in an unhealthy and potentially life-threatening way.
An investigation by BBC Watchdog found there are no restrictions to prevent people buying large quantities of laxatives, despite repeated warnings from charities about widespread abuse by young people who use them to lose weight.
BBC Watchdog sent 14-year-old actors into 25 branches of Tesco, Sainsbury's, Morrisons, Asda and Boots to buy three boxes of stimulant laxatives.
None of the actors were stopped or questioned by staff in any of the stores, despite buying a minimum of 60 tablets each time.
The eating disorder charity Beat said it warned five years ago that people, including teenagers, were using laxatives to lose weight.
Some 80% of those who had been affected by an eating disorder said they had misused laxatives in order to lose weight, a survey by the charity found.
Laxative abuse can cause diarrhoea, dehydration and electrolyte imbalances that can lead to kidney failure.
It can also damage the liver and potentially damage the heart, leading to heart failure and death.
Beat is calling for restrictions to be put in place by supermarkets and retailers to reduce the ease with which young people can buy laxatives in large quantities.
It has suggested a minimum purchase age of 16, a maximum pack size reduced to 10 tablets, sales restricted to pharmacies and a warning label on packets.
Beat chief executive Susan Ringwood told BBC Watchdog: "Young people don't have to look very far to find this information. They turn to the internet. That's the first place they go to.
"We've seen a 30% increase in calls to our helplines over the last year where people have mentioned overusing laxatives in unhealthy way. And that does include a significant number of young people as well."
Tesco, Asda and Morrisons said sales of laxatives were made in accordance with regulations set by the Medicines and Healthcare Products Regulatory Agency (MHRA).
Boots UK director of professional standards, Steve Banks, said: "As a responsible pharmacy-led retailer we're concerned to hear from BBC Watchdog about Beat's report that there has been a rise in incidences of customers misusing laxatives.
"Providing safe access to medicines in line with current MHRA regulations, with the relevant healthcare advice and information available is at the heart of what we do."
He added: "We want to provide support to the small minority of customers who misuse these products, while making sure that the majority of customers who use these medicines appropriately can continue to do so.
"In response we will be providing further training for our healthcare colleagues so they can better identify possible signs of misuse and offer further advice and support, including signposting customers to further sources of help.
"On an issue like this it's important that manufacturers, retailers and regulatory bodies work together to look at how to respond and we would welcome being involved in a wider conversation on this topic to understand what further action could be agreed."
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Sainsbury's said it acted within the law around the sale of laxatives, and would be reinforcing the message given to employees to be vigilant for bulk purchases at meetings this week.
The MHRA said: "Most laxative medicines are used by patients safely and in accordance with the instructions for use on the patient information leaflet (PIL), however we do recognise that some patients misuse or abuse them.
"The Patient and Public Engagement Expert Advisory Group (EAG) which reports to the Commission on Human Medicines (CHM) has recently reviewed the patient information for non-prescription laxatives and has recommended that stronger warnings should be added emphasising that taking laxatives regularly for a long time is harmful and they do not aid weight loss.
"We are currently working with companies of stimulant laxative products to introduce these updated warnings which should provide consistency across the range of stimulant laxative products available.
"We will continue to monitor the safety of non-prescription laxatives and will take further action if necessary."