PARIS -- A prestigious medical journal has issued a warning over the use of Botox, the anti-wrinkle treatment that is spreading like wildfire among celebrities and the well-heeled, AFP reported.

Botox -- botulinum toxin, derived from the bacterium clostridium botulinum -- is a viciously powerful neurotoxin that at high doses can cause fatal paralysis.

At low doses, it is licensed for use to treat neurological conditions such as facial tics and muscular spasms in the neck, back, fingers and wrists.

But it is starting to move out of the clinic and into unlicensed areas, driven by a frenzied demand by men and women who want to look younger.

By paralyzing muscles that cause wrinkles in the forehead, it can smooth out age and worry lines. Global sales of Botox have risen from 25 million dollars in 1993 to an estimated 430 million this year, driven especially in the United States, the home of "Botox parties."

The craze has been driven by media coverage and endorsements by celebrities in the music and movie business. One person who has acknowledged having used it is Sir Cliff Richard, the veteran British singer.

In an editorial published in Saturday's issue, The British Medical Journal (BMJ) sternly warns that many aspects of Botox's long-term impact on health have never been explored, especially on the nervous system.

"Robust evidence for the action of botulinum toxin on sensory neurons is lacking," according to the editorial, written by Peter Misra, a consultant at the National Hospital for Neurology and Neurosurgery in London.

"Animal experiments have shown that botulinum toxin affects the transmission of afferent nerves," he said, referring to nerves that relay signals from the limbs to the central nervous system.

Other studies, among rats, have shown that Botox can block the release of nerve-signal chemicals called neurotransmitters in rats, he said.

"It is easy to forget that botulinum toxin is a potent neurotoxin and that its very long-term effects are still unknown," he warned.

The BMJ is the weekly organ of the British Medical Association (BMA).

Botox took a significant step towards moving out of the celebrity sphere in May, when the British high street group Boots started to offer injections, administered by doctors, at three stores for 200 pounds (300 euros, dollars).

Botox treatment is not permanent -- follow-up injections are needed a few months afterward -- and those who receive it sometimes complain their faces look frozen and inexpressive.