The popular anti-wrinkle drugs Botox and Myobloc have been linked to some deaths among children, mostly those with cerebral palsy who were treated for limb spasms, the Food and Drug Administration warned Friday.

The FDA has never formally approved the use of the drugs for muscle spasms, although other countries have; yet some U.S. doctors have chosen to use it for this purpose.

The drugs use botulinum toxin, which blocks nerve impulses to muscles, causing them to relax. The FDA warning notes that in some rare cases, the toxin has spread beyond the injection site to other parts of the body, resulting in such problems as paralysis of muscles used for breathing, leading to death.

A spokeswoman for Allergan Inc., the manufacturer of Botox, says that children with cerebral palsy typically receive far larger doses injected into their leg muscles than the doses given to adults seeking facial wrinkle care.

The FDA said problems with Botox and Myobloc, which is manufactured by Solstice Neurosciences, might be caused by overdoses. It said there is no evidence that adverse reactions are related to defects in the products.

The FDA is currently reviewing the safety of both drugs, using data from clinical studies submitted by the drugs' manufacturers, as well as post-marketing adverse event reports and medical literature.

The FDA said Friday its investigation is still in its early stages. For now, the agency said doctors should warn all patients receiving a botulinum toxin injection to seek immediate care if they experience difficulty swallowing or breathing, slurred speech or muscle weakness.

Health Canada is also reviewing safety information on Botox, which is approved in Canada for cosmetic purposes, as well as for the treatment of spasticity or muscle stiffness.

"Canadians can be confident that after a thorough review, Health Canada will take action, if necessary," Health Canada spokesperson Carole Saindon told CTV News by email last week.

Friday's warning came two weeks after the U.S. consumer advocacy group Public Citizen petitioned the FDA to strengthen warnings to users of Botox and Myobloc, citing 180 reports of U.S. patients suffering complications, including 16 deaths.

Public Citizen wants the FDA to require Allergan to write to doctors warning them that botulinum toxins marketed in the United States have resulted in "hospitalizations and deaths."

It also wants the FDA to order that Botox and Myobloc carry the strongest possible warning, highlighted in a "black box." The FDA should also require a consumer-friendly guide explaining the risks that would be handed out by doctors when the drug is injected, the group recommends.