The popular antiwrinkle drug Botox and a competitor have been linked to dangerous botulism symptoms in some users, the Food and Drug Administration warned, in some cases so bad that a few children given the drugs for muscle spasms have died. The warning includes Botox, a wrinkle-specific version called Botox Cosmetic and its competitor, Myobloc, drugs that all use botulinum toxin to block nerve impulses, causing them to relax. In rare cases, the toxin can spread beyond the injection site, paralyzing or weakening the muscles used for breathing and swallowing, a potentially fatal side effect, the agency said. Botulinum toxin is widely used for a variety of muscle-spasm conditions.