Federal health regulators last week sent warning letters to 18 websites selling counterfeit or unapproved versions of Botox and similar injectable drugs 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 also is approved in the United States 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.