A new study raises concerns that using the common cosmetic treatment Botox might be dangerous.

Researchers injected Botox, which contains the deadly botulism toxin, into the whisker muscles of rats. Traces of the poison traveled to the rats' brain stems, where they were detected as long as six months later.

Botox is made from the highly lethal bacteria Clostridium botulinum, although very small amounts are used for beauty treatments. It works by temporarily weakening or paralyzing muscles and nerves to smooth out the skin.

The drug has a somewhat checkered history. Last month the US Food and Drug Administration (FDA) warned that Botox and the similar drug Myobloc caused severe muscle spasms in children with cerebral palsy, a use not approved by the FDA. Even earlier, the healthcare consumer advocacy group Public Citizen asked the FDA to warn strongly against the use of Botox and Myobloc, after 180 reports were received of users having fluid in the lungs, difficulty swallowing or pneumonia, which resulted in 16 deaths.

The author of this latest study, Dr. Matteo Caleo of the Institute of Neuroscience in Pisa, Italy, says the toxin moved from one hippocampus, which controls long-term memory and spatial navigation, to the hippocampus on the other side of the rat's brain. This implies that there could be some transmission of the toxin to the central nervous system.

The study authors say that although rats and humans have a different physiology and their responses to a particular drug may or may not be similar, the results are a concern and more research needs to be carried out.

The study is published in the April, 2008 issue of the Journal of Neuroscience.