U.S. health officials are warning doctors and patients about potentially deadly risks of using anti-wrinkle drug Botox and similar drugs for unapproved uses to treat muscle spasms.

The Food and Drug Administration says Botox and two other injections have been linked to rare botulism symptoms, particularly when given to children to help relax uncontrollable muscle movements.

While Botox is best known for clearing wrinkles by paralyzing facial muscles, the botulism-based drug also is widely used for muscle-spasm conditions.

Regulators say the majority of deaths are in children with cerebral palsy taking the drugs for spasticity in their legs.

Botox and competitors Myobloc and Dysport will carry a boxed label warning, the most serious type available.