Associated Press (AP) -- 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.

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. In each case, the FDA said the companies were offering unofficial or mislabeled versions of Botox-like drugs that haven't been approved by the agency. In addition to the original drug, introduced by Allergan in 1989, the FDA has approved several competing versions.

FDA-approved Botox drugs carry the agency's most serious warning, a boxed label alerting doctors and patients that the medications can cause serious or life-threatening side effects.

In rare cases, the toxin can spread beyond the injection site to other parts of the body, paralyzing or weakening muscles needed for breathing and swallowing. Signs of botulism include difficulty swallowing or breathing, slurred speech and muscle weakness. Those symptoms can occur several hours after an injection.

In a press release Wednesday, the FDA said patients should only receive the drugs from health professionals who are licensed and trained to administer them. Patients experiencing signs of botulism should "seek immediate medical care," the agency noted.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute's Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.