Medical experts have warned that the adhesives used for eyelash extensions can carry risk of infection, allergic reaction and even cause the natural lashes to fall out.The popular beauty procedure, which sees individual synthetic lashes attached to the wearer's own for a fuller look, was the subject of an investigation by Consumer Reports.The doctors consulted in the report say that many of the adhesives used by beauty salons contain formaldehyde, which some people are allergic to.Dr Neda Shamie, an associate professor of ophthalmology at Doheny Eye Institute in Beverly Hills, California, told ABC News: 'Any chemical exposure to the cornea being so fragile in some ways or susceptible to scarring and irritations and infection, it could be harmful.'But even if one is not allergic to formaldehyde, eyelash extensions can cause an infection known as madarosis, explained Philip R Rizzuto, MD, secretary for communications for the American Academy of Ophthalmology.'Sometimes just the irritation from the glue used can lead women to rub or tug on their lashes,' he said, adding that many ophthalmologists have reported a rise in patients with problems relating to eyelash extensions.Consumer Reports warns that the eyelash extensions can also trap dirt and bacteria, heightening the risk of infection.And the UK's College of Optometrists in England warned that the procedure can even cause permanent loss of eyelashes.'Repeated use of eyelash extensions can cause traction alopecia, a condition where the hair falls out due to excessive tension placed on the hair shaft,' a press release stated.'As a result, this can damage the hair follicle, which can slow down and even cease production of hair.'Indeed, the risks associated with the treatment were made widely apparent when Broadway star Kristen Chenoweth appeared on The Late Show with David Letterman wearing dark glasses in February.She told the host: 'Something bad has happened. I’ve got eyelash extensions. Here’s the problem: The glue has formaldehyde in it, and I’m allergic. I swelled up and I’m sneezing. . . It looks like I have lips on my eyelids.'The Association for Damage-Free Eyelash Extensions defends the procedure, however, telling ABC News that the 'adhesive should not contain formaldehyde,' and 'hypoallergenic adhesives are available.'A spokesman said: 'Properly applied eyelash extensions are not dangerous.'