By MATTHEW PERRONE

U.S. health regulators on Wednesday sent warning letters to 18 websites selling counterfeit or unapproved versions of Botox and similar injectable drugs commonly used to smooth wrinkles.

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The Food and Drug Administration said it was taking action after receiving reports of injuries in connection with the products, including toxic side effects.

Botox is a diluted, purified form of botulinum, one of the most toxic substances in the world. The ingredient works by temporarily blocking nerve signals and causing muscles to relax. While most famously approved for cosmetic use, Botox is also approved in the U.S. for a number of medical conditions, including muscle spasms, eye disorders and migraines.

The FDA warning letters mostly went to cosmetic websites.

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